Apparently there's a vaccine for chicken pox but it's not administered in the UK often. Seems to be more commonly used in the US though, not sure about CanadaWith chicken pox you keep the sick child at home you don’t isolate the entire class! Or traditionally we had chicken pox parties!
Either way, and as my brief Google search suggests, schools/nurseries may (or should? Someone here probably knows more) notify parents that someone has chickenpox and to watch out for symptoms. I'd assume the same would be done with monkeypox. Think the bigger problem is in countries where schools don't seem to have the same/similar approaches to keeping parents informed and children healthy though