We now have to make a PA on US flights that mentions that pax over 2 are required to wear a mask as per US federal law.
To be fair, personally I would overlook a mask-less 2 year old however rules are rules and some crew may not.
All I can advise is get them used to wearing them, and at least make an effort to have them wear a mask onboard, however I’m sure if they really won’t then you won’t encounter a problem.