In the near term COVID isn't that likely to make them really sick but a non-negligible % of kids appear to get MIS-C or weird long-term effects like diabetes. If we can extrapolate much from adults getting the kids vaccinated will go a long way towards reducing those.
Sure, and diabetes around 0.05%, and then the assortment of cognitive, respiratory, and cardiac symptoms that are less life threatening but more common. AAP has a reasonable overview:
There's a fair amount of uncertainty but stack it up the likely negative outcomes and I wouldn't let my unvaccinated kid catch COVID if I had a reasonable way to achieve that.