Children age 2-5 are learning faces and how to speak. If they can't see each other's
faces and mouths, they don't learn a basic part of human interaction.
Think a little more broadly about "harm".
Maybe they'll end up being more understanding and kind to autistic people who have troubles with reading faces or maintaining eye contact regardless of being raised up with masks or not. That would be nice.
The upside of having a developmental disability is that you have compassion for other people with developmental disabilities? Ok.
Yes, obviously. A lot of trauma suffered by people on the spectrum does not come from their autism, but from how other people assume that everyone else is like them. Reading faces is not an essential skill that everyone needs to learn in order to operate in a healthy society, the only reason people who lack that skill have troubles is because it is expected from them by others to have it. If we raise a generation where a significant percentage of people don't develop this skill very well, we may end up with a society that doesn't expect others to rely on skills they haven't developed, making everyone's life easier.
If I could choose between a society where 80% of people can read faces, vs. one where 90% can read faces, I'd choose the one where more people can read faces.
It makes everyone's life easier when more people can read each other's faces.