Wallpaper is such a great idea for kids rooms. It need not be expensive either - one feature wall is all you need. This will actually make a much bigger impact than papering all 4 walls.
Here's some inspiration. 5 rooms with divine, gender neutral wallpaper ideas.
The first 2 rooms are from www.fermliving.com:
Animal print and Half Moon
The below wall paper is from www.sianzeng.com