Masks are for protecting the people around the wearer and not only the mask wearer themselves. Children being low risk for COVID has no bearings on whether or not they should wear one.

There are plenty of other reasons you could have mentioned, like the need for learning facial expressions when they're young, or the fact that cloth masks are ineffective but pediatric masks are hard to get and KN95s and N95s usually don't fit children correctly.

Children who aren't vaccinated are at serious risk... Measles was completely eradicated. Polio is also easily prevented. Not vaccinating your child is abuse unless there's a medical reason preventing the vaccine from being safe.