Okay, I don't sew, but I know I've seen instructions on how to get ruffles on the behind of a onesie. I did a quick search, which turned up NOT what I remember seeing. However, I found a couple other things that might be helpful: http://fashionmamanista.vox.com/
(tute for attaching ruffles to the bottom of leans, it seems like you could just alter the length of the fabric) https://sewing.patternreview.com/cgi...iew.pl?ID=1759
(a different way to get the ruffle)
I hope those help! Good luck!