You can stamp with craft ink directly on the onesies if you wash and dry it (without fabric softener) and then heat set it with an iron (no steam). Then put it in the dryer if you want to set it a little extra. Another option I have is done is stamp on an iron-on transfer with a classic stamp pad and then iron-on per the directions. The only problem I have had with this technique is it cannot be a solid image. My colors have bled after being washed then. Otherwise, contact a local Stampin' Up! rep. We sell staz-on ink!
Hope this helps you!