I like Jan's idea also -- the piano key border would make the quilt more informal and kid-friendly. A railroad track border is also a great idea!

I love the Railroad Track Border idea because it is for an older toddler who loves trains.
We as quilters and adults think of making the quilt pleasing to our eye with balanced colors...but a child, they just want to have fun playing and snuggling with their quilt. Any choice you make, the quilt will be loved and enjoyed!

If it is large enough I would just bind it in black. It looks really good to me like it is and a simple black binding would end it off.

Update - Train quilt
 

I loved the idea of the piano key border, but when I made 6-9 inches and added it to the design wall, it just didn't work for me. This is what I decided on. I will probably bind it in black but wait to see when i reach that stage.

Again thanks for all the ideas.