Anna, that's a great question. I don't have a product like that, so I haven't researched it, myself. If the onesies are 100% cotton, they are probably considered ok by the CPSC, based on the link above.
However, each seller should research and gather info for themselves. I am definitely not a lawyer!
For my plush, as an example, I am using cotton materials from a manufacturer from whom I was able to download and print testing results and certification. I feel I can sell these within the law.