Another option for decorating onesies at Jacklyn's shower was using stamps and fabric paint.
I found these Scribbles Stamps:
and bought some Tulip Fabric Paint in various colors:
Using a paint brush, you can apply paint to the stamps and stamp onto the onesies. Be sure to use a small piece of cardboard in between the front and back of the onesie so you don't have any paint bleed through. Stamp on!