What Dina said would work fine.
You can also cross hatch - draw a diagonal line from corner to corner both directions then using those lines as a guide sew parallel lines about 3" away from the first ones. That makes diamond shapes in the quilting which looks cool.
I've made a few quilts for Quilts for Kids and I've done some as Dina said and some using the cross hatch.
Really cute quilt. I know some child will love it.