1. Most masks for children fit extremely poorly. I doubt they do much filtration of incoming air. Maybe they’re useful for source control, especially for pathogens that spread through large droplets (i.e. maybe not Covid).
2. They do have one major obvious effect: when worn even remotely correctly, they keep kid fingers and hands away from kid mouths and noses.
So I expect them to be quite useful at protecting kids from pathogens that are contracted by collecting the pathogens on one’s hands and sticking them in one’s mouth or rubbing one’s nose. And they may also have a strong source control effect for pathogens that come from kid mouths or noses, get on kid hands, and spread onto surfaces.
These are not descriptions of Covid! But there are a lot of diseases that kids get frequently that aren’t Covid. I bet they substantially reduce transmission by the fecal-oral route, for example. And they may help quite a bit against the kinds of viruses that kids actually get from outdoor playground, especially if the kids wash or sanitize their hands after playing.