I love that wallpaper! Then again I'm not 3 I suppose. I think the room you've planned sounds lovely and it makes sense to keep the basics of the room neutral then it's easy to update later when their tastes change as they inevitably will.
That said I don't think there's much point trying to impose adult tastes on a child. My DD had her room decorated for her birthday a few years back, I thought she loved it but can see now that I took her ideas and interpreted them my way
.
I recently gave in and said OK it's your room, do what you want with it (not crossly, just accepting that she's getting older and knows her own mind). We haven't redecorated but every available inch of wall space is covered in posters, there are things hanging from the ceiling, scarves pinned up, tat accessories on every surface and you can't move for cushions. It's so far removed from my idea of a tasteful preteen room but I've found that I actually love it - you walk in and just know that it's DD's room - everything in there expresses her - song lyrics, quotes, photos, drawings, mementoes.
It's messy, cluttered and nothing like the carefully designed, stylish room I can see in my head but DD loves it and it's her space. I've come to realise that at nearly 13 she may only be living here for 5 or 6 more years and I want her to be happy with her room even if her idea of how it should look is very different from mine.
I know your DD is only 3 but it goes so fast, maybe worth giving into what she'd really like and being glad it's only one room that you can close the door on :).