There is such a thing as an afterthought buttonhole where you pick up the stitches around the place you want the hole and bind them off. I suspect, though, that a baby sweater has just a yo buttonhole. Maybe you can just pull a strand of yarn to each side and stitch it in place to create a hole?
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry