I went to the Etsy site and typed in Children's fabrics and cam back with 250 pages of stuff here are some of the names of shops that I found fitting your criteria : Undercoverquilts; the750fabricshop;Sharonsringsnthings; and many more - take a look.

I use flannel. If I know the child has a special interest, such as trains or princesses then I use that on the back. My nephew was crazy for Thomas the Tank, so I used trains and my niece was in love with all the Disney Princesses. There are so many wonderful flannels out there, I don't have any trouble finding something to compliment the front.

Carolina Cotton Co. has a children's section you might want to look at.

I also like something calmer for the back.