There are many simple quilt patterns you could use: Rail Fence, Warm Wishes, Jelly Roll Race (baby quilt size), log cabins of every configuration, chevron quilts made of HSTs, and on and on and on. Just choose light green and blue fabrics. And then there are all the ways you could use scraps!
But here's a different idea: My "signature" baby quilt has become a very simple, straightforward I Spy. While it took me a little while to collect all the fabrics I have today (during which time I cut up a number of multi-animal prints, etc. - and still think that's a great way to go), I now have 80+ unique images that I draw from. Some are fussy-cut; some are not.
I cut 4 1/2" squares. Each quilt contains 80 squares. Lately I've been backing the quilts with Minky (and no batting) and tying them. Needless to say, there are many alternatives!
My daughter is a psychiatrist who specializes in children, and she was the one who encouraged me to produce these quilts. Another friend, a life-long teacher, concurred, adding that such a quilt could be an important vocabulary builder.
Just my $.02