What visa options are available for an infant in Thailand while parents hold non-ED visas?

Don't worry about it. Children that young cannot be fined or banned for overstay.

The next time you might worry about it is if you want to enroll them in school. Some schools require the students to not be on overstay to register. But that's a few years away and you can easily clear their status by exiting and entering the country again. They'll get an overstay stamp but it won't mean anything and they can re-enter and be back in good standing.