I'm going to visit one of my friends next week, who is an artist and a horse lover.
I'm not sure if she could do anything appropriate for a child's room, as her work is usually more 'adult', but I will ask her about it.
Here is her website, so you can get a feel for her style -- Christa Rosenkranz & Ckranz Studios
She doesn't have any of my personal favorite works of hers, on there though :\ Just a small sampling.
Under 'Art and Illustrations', those two horse paintings are actually large murals that she did for the equestrian camp we used to work at. They were wanting a more 'cartoon' than realistic style. She also did another larger mural, for them, of a herd of horses, that was more of an 'artistic' style and was amazing!
I think she could do some nice 'fairy tale' feeling pieces that would be great for a little girl's room.
I'm not sure if you're comfortable posting pictures of your daughter here, but if you wanted to pm me a couple pictures of your daughter's horse, or whatever you were thinking you might want painted, and I'll show them to my friend :)