Your paranoia has me worried now

... I have printed plenty of onesies and lots of child clothing and have not worried about this. Any manufacturer defects (buttons coming off, threads coming loose) would be on the the manufacturer and you wouldn't be responsible. Make sure that the shirts/onesies are cured correctly and you should not have to worry about the ink that you are using.
If you are still worried... draw up a short legal disclaimer that explains that you do not take responsibility for manufacturer defects and any damages resulting from those defects.
Brian