Cute quilt! I think I would go with a lime green fabric (like the light lime green pigs or the handles on some of the tea pots).
Yellow could also work, but with the slightly darker shades in your 2nd quilt, I'd probably pick more of a gold (like the butterfly fabric) rather than the brighter, purer hue of yellow you used to bind the original quilt. I personally like to back my children's quilts with novelty prints -- aliens, fire engines ... something that is a busy print that would connect nicely with my binding & thread color and that incorporates at least some of the colors from the front of my quilt. And, depending on the child, sometimes I'll pick out a handful of fabrics from which to choose in the store & let the recipient pick the backing fabric. My niece gets so excited every time she sees her quilt with the backing that she picked out herself (it's pink flowers -- go figure). She proudly tells everyone that it's the fabric that she "choose-d". She was 3 at the time; she's 4 now.