In your opinion, what is wrong with the windows shown in the ast elevation you show?

I am worried they are too low and the kids can climb/ fall out when they are open.

Could you consider something like one of these, where two narrow operable windows flank a large fixed pane window? There are unobtrusive window locks that would enable you to open the side windows enough to enjoy a breeze but not enough for small children to climb out or fall through.

https://www.houzz.com/photos/last-house-on-the-left-contemporary-dining-room-boston-phvw-vp~1720493

https://www.houzz.com/photos/integrity-windows-and-doors-transitional-living-room-phvw-vp~91244996

https://www.houzz.com/photos/seabend-traditional-home-office-boston-phvw-vp~1192477

JP Haus thank you for narrow window idea, I did not think of that at all!

Have you had any experience with children falling out of windows, or know of anyone that has?

Bedrooms will be required to have egress windows that can open wide enough for a person to pass through the opening, but the sill can be set to a height that will inhibit small children from falling out.

I had a client that had a habit of falling off his roof, but we fixed that by building a deck and railing above the roof where his habit persisted; that and reduced his consumption of alcohol while on the roof/deck.

Thankfully not yet! My toddler and 1 year old seem to do all kinds of daring things so I think that's where the root of my worries come from. I also caught my small niece standing on a 2nd floor window sill smooshing her face into the screen where the window was half way open. I want to be able to relax in my home but maybe I just need some serious child proofing done! I hope that they never make it up to the roof like your client!

For a panoramic view the individual windows should be much wider which is best achieved with double-hungs. Its not difficult to clean the exterior screens or the glass and they look much better without interior screens.

Window safety is provided by code required "window fall prevention devices" that stop the windows from opening more than 4" but can be manually released.

Good ventilation is provided by opening the top and bottom sash 4".

Avoid fixed widows that are difficult to clean.

At the ground level casement windows can obstruct movement at a terrace or deck.

Casement windows in adjacent rooms can reflect sound from one room to the other surprisingly well.

I do not avoid large fixed picture windows because they're difficult to clean. Even when lightly soiled from rain a beautiful view without vertical interruptions is continually rewarding. It is a focusing pleasure whenever you enter the space. A squeegee and extension can allow that pleasure without undue trauma. I have 18 large fixed glass windows.

FWIW I wouldn't make a long term decision about what windows to use in my house based on the very temporary and soon-to-change behavior of a couple toddlers. Get some baby gates to put on the doors to those rooms if it concerns you, and/or don't open the windows until they're old enough to not inadvertently fall through them (although I'd also argue that a 1 year old and toddler should always be within sight so this opportunity shouldn't exist). By the time you finish your addition, this behavior may be long past anyway.

I also agree with @dan1888. We have 4 large picture windows that I can't clean from the inside, and I wouldn't change it. I get all the windows professionally cleaned once a year (even the ones I can clean myself) and I find it's worth the trade.

I agree with double hung for floor level windows, and large fixed/picture windows above.

For our large windows we chose a sliding window option and will place childproof stoppers in the track until our children are old enough to know better. For the single, floor level, windows we chose casement style, the only thing I do not like about those is that the screens are installed on the interior and I am afraid the kids are going to tear them. We will leave them removed until the windows need to be opened.

If the project is in the US, the building code will require one bedroom window to meet the emergency escape and rescue opening requirements and the windows on the upper floors will have to meet the fall protection requirements.

Another vote for double hung with picture windows above.