No specific ideas, but what a fun little room!

ok- maybe stick on glow in the dark stars for the ceiling.

Shop for fun and colorful bedding to start. Make sure she has hanging bars where she can hang up her clothes.

What a fun space!

Paint glow in the dark stars and constellations on the ceiling, even just little dots to light up the night sky. I sleep in my niece’s room when I visit and I am charmed by the painted constellations on the ceiling. They are white in daylight, so don’t show. My 17 year old niece has redecorated the room in many ways, but the stars remain.

What are the room’s dimensions? Include where the ceiling changes height and how high the knee walls are. Is the door to the side in the first photo closet space?

My only concerns in an old house in the top of the building would be temperature. Before decorating, be sure that there is adequate heating and cooling of the space, and if not, consider adding some blown in insulation in the the wall and ceiling cavities and some type of insulation around the knee wall space. I also might well get a fancy mesh metal radiator cover to protect from direct contact with the radiator.

edited to add that It’s fun that Cyndy and I had the same idea and crosssposted.

How charming! I would also be concerned with heating and cooling issues.

Love those windows .... I’ve never seen anything like them. Lucky 6 year old ! Include her input , it WILL change in a few years. Have fun.

Any of those comforters would be adorable in this room.

How about this, if there's room? https://www.amazon.com/Play-Tent-Boys-Girls-Astronaut/dp/B01F9MO9PU

It makes me laugh that it is marketed as a play tent for boys, and the girl's equivalent is a princess castle. My 3 year old granddaughter has the princess version, but it is usually a submarine, occasionally a space ship. Never a castle. It is sturdy and roomy, and easy to move from one room to another, or outside, when friends come to play. She also has an Ikea crawl tunnel which is the "air hatch" for either space or water.

What does SHE want?

Get her involved. Let her have the fun of putting it together (with some guidance from you). Let her pick the color. Let her pick the décor subjects.

This is not a permanent decision. This will last 3-4 years until she gets a bit more grown up, no matter who designs it.

https://www.amazon.ca/Remote-Control-Décor-Night-Light/dp/B000EUHKUE/ref=sr_1_1?keywords=uncle+milton+moon&qid=1554379296&s=gateway&sr=8-1

https://www.etsy.com/ca/listing/588696513/deer-art-poster-moon-river-fantasy?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars&ref=sr_gallery-1-22&organic_search_click=1

https://www.etsy.com/ca/listing/226887325/bunny-rabbit-stars-night-sky-galaxy?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars&ref=sc_gallery-1-10&plkey=b8baa00ec3a1850b402b6ba592b2e6c994ed96ee%3A226887325&sca=1

https://www.etsy.com/ca/listing/593433947/fox-art-print-the-little-prince?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars&ref=sr_gallery-1-43&organic_search_click=1

https://www.etsy.com/ca/listing/612043845/eclipse-celestial-space-cat-bicolor-pink?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars&ref=sr_gallery-2-48&organic_search_click=1&frs=1

https://www.etsy.com/ca/listing/503587841/watercolor-painting-whale-shark-painting?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars+whale&ref=sr_gallery-1-2&organic_search_click=1&bes=1

https://www.etsy.com/ca/listing/671449729/small-between-worlds?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=art+print+stars+whale&ref=sr_gallery-1-3&organic_search_click=1

Is she girly? The love of outer space and girly don’t work in my head. I would for sure involve her in every choice what will happen is that you will start to see who she is. I always kept wall color really neutral so that each time the favorite color changed I did not have to paint. My Dad put up huge cork boards in each of our bedrooms so that all those pictures and other things could be displayed without tape or holes in the walls. I would start by getting her choice of bedding and then work from there I had an attic bedroom and I loved it .

https://www.pinterest.ca/pin/88805423873392077/

what color does she want? pink? blue? green? purple? white? what styles does she like if any? flowery? retro? or only space? even at this age they have some sense of what they like.. then make up some printed out docs with those options and let her choose.. mood boards for a 6 year old.. what is your budget? what do you have that you can use? girly outer space - you can find stick ons for the walls at etsy.. paint the ceiling darker and use the glow in the dark stars.. get her a space suit costume and take a photo of her that you can have printed and framed or made into a life size cutout to stick on the wall.. i am a mom of boys.. so it was cars, army and soccer.. it would have been nice to have a girl to decorate for.. have a custom window shade made with stars and planets that lets the light filter through.. they also have the space night lamps to shoot the stars and planets onto the ceiling and through the room.. if this is hers only make it her haven - with an extra place for friends who will want to stay over..

doesn’t even have to be pink, what makes exploration, science and nature unfeminine?

https://www.spoonflower.com/en/fabric/6530784-space-cats-fabric--cat-cats-design-cute-cats-kittens-kitty-design---black-by-petfriendly

https://www.spoonflower.com/en/fabric/7551187-floral-space-by-crystal_walen

https://www.spoonflower.com/en/fabric/3194629-space-fantasy-by-siya

https://www.spoonflower.com/en/fabric/6150348-space-planets-by-jadegordoni 

https://www.spoonflower.com/en/fabric/7912066-belka-strelka-space-by-korybing

https://www.spoonflower.com/en/fabric/6388688-solar-system-buddies-by-bashfulbirdie

https://www.spoonflower.com/en/fabric/430690-solar-system-banner-by-wildolive

https://www.spoonflower.com/en/fabric/4833936-constellations-origami-geometric-animal-astronomy-stars-night-sky-navy-blue-kids-nursery-baby-print-by-andrea_lauren

https://www.spoonflower.com/en/fabric/6052591-star-map-dark-bue-by-emeryallardsmith

https://www.spoonflower.com/en/fabric/4795926-moon-phase-black-white-abstract-dark-night-sky-geometric-by-canigrin

https://www.spoonflower.com/en/fabric/8327030-calculating-moon-by-vinpauld

https://www.amazon.ca/Papa-Please-Get-Moon-Miniature/dp/0887081770/ref=sr_1_1?crid=3NGK36Y73JNA6&keywords=papa+please+get+the+moon+for+me&qid=1554408140&s=gateway&sprefix=papa+please%2Caps%2C150&sr=8-1

Not a Pro.

I love the color of the walls in this room and what a space for your little girl.

I would pick a solid colored comforter and add fun space pillows. Glow in the dark stars on the ceiling. Maybe a peel and stick wall paper.

Next year she will want something else. Keeping the comforter a solid color that will help make changing things up a little easier.

Following.....

Saw this and thought you could do something similar on your slanted walls with paint It nstead of wood framing. But put in constellations like the Big Dipper and Orion's Belt

Edgewood - Fairy Tail Bedrooms · More Info

For glow-in-the-dark stars or stickers, etc., go to your local science museum's gift shop (after you take her through it). The Franklin Institute in Philly (and probably the Please Touch Museum) has that kind of stuff. Make it a day.

Very cute

This room makes me nostalgic for my childhood bedroom! I had an attic room in an old Sears Roebuck catalog house. It was miserably hot in the summer but I loved it (good for you for insulating!). Past experience in an old house would suggest some sort of fan for better air circulation, if there's no central air.

I got to design parts of the room myself. Picking colors etc. My dad built a tree around the chimney for me with bookshelves. Bookshelves are always useful, whether the kid is a bookworm or not. And yes, I had glow in the dark stars everywhere on the angled ceiling!

ETA: Speaking of old home experiences...radiators love to eat small toys. A radiator cover would be a good idea to keep your daughter's toys from getting stuck (and for safety, for that matter). You could then use that as the base of a set of shelves that goes up the wall between the windows.