Like some of the previous posters I recommend quilting every 2-3" for children's quilts that get a lot of use (and washing!). I also agree that putting Xs through the blocks plus SID would provide a lovely quilt plus make it very washable. Don't worry about the thread color not "matching". After a quilt is washed it will crinkle up and hide a lot of the stitching. Just use your walking foot, a nice thread (a 50 weight cotton or So Fine polyester would be easy to work with), and go for it. Your quilt is really cute, and it will be greatly loved.