Do you know how to crochet? If not it is a bit harder. I saw this question the other day but didn't have time to go to the pattern and thought it was for an adult. If I'd known it was for a baby I'd have said something sooner.
Here is a little sweater (and hat) that I added a crocheted border to.
Something like this could work. If you have Ravelry I have a more close up picture of it there. For your little sweater since it already has a finish on the cuff and waist band I'd just start it at the lower right hand front edge and work in one continuous row up the front around the neck and back down the other side. Just attach the yarn with a slip stitch and then chain a few 3-5 depending on the look you want (experiment on a swatch) and then skip 2 or 3 stitches and slip stitch (or single crochet) it down and do it again.
If you still haven't done this I know of another pattern that has a simple little edge trim that I could look up for you.