Sometimes rex lose their curl as they age (but the whiskers should still be wavy at least), so it could be that your parent rats were poor rexes and then the baby got genes from both and is showing double rex.

True hairless is recessive and would have to have been carried by both parents... But I don't think they are born with any hair (and the baby looks double rex, not hairless).

Are either of the parents' whiskers wavy at all (can't tell from the pics)? My money would be on them being poor rex and not visibly showing at this point. If it is not that, then I'm not sure what happened.