Really I think we need mor einfo. How old is she what are her interests ? Akids bedroom needs to reflect who they are and what they like I am always hesitant to use wallpepr in a childs room since hwta thye like can very often be a yearly thing until they arequite old. Maybe post a pic of her room now that will give us a clur into her space at least. As for wallpaper there are types that can be removed easily This one is really cool and you could do some gold leaf on it . It is removeable too

https://www.target.com/c/wallpaper-home-improvement/-/N-5xtsdZgup4zc5xktoZgup4zc5zk8t?type=products

Always better to see the room/to know a bit more

Does she like flora and fauna? Or something pictorial? Or funky?

Maybe mural would work better on one wall only?

Meanwile heard good things about this site https://tempaper.com/. All removable

(that's really gold and pink, but they have so many designs there..check them out https://tempaper.com/bird-watching-pink-metallic-gold-bird-removable-wallpaper)

or Etsy is good. This lady's shop has very positive reviews. (chose it by the colors only, she has more than 50 patterns https://www.etsy.com/listing/782428452/wallpaper-leopard-dusty-pink-unique?ref=shop_home_active_29&frs=1)

But there are many shops there. You can search "wallpaper" or make it more detailed to get more precise results.

Check Anthropologie-they carry more and more wallpaper, and I love them for reviews, so one can make up his mind more easily. People usually write detailed reviews there. Service is good too, as a rule.

https://www.anthropologie.com/wallpaper