Three children, and masks that are simply part of their game. That is all the poem gives us, and its power comes from how little it says. The ellipsis after "Three kids" works like a pause while the speaker looks, as if noticing them for the first time.
The poem doesn't tell us what kind of masks these are, and that openness matters. Depending on the reader, they might be Halloween costumes, face coverings worn for health reasons, or something else entirely. Judging by the wording, what interests the speaker is the children's ease. For adults, a mask might carry weight or worry. For kids, it is just another toy, "just part of their play."
The mood is quiet, gentle, and a little wistful. The speaker seems to watch with affection, perhaps with some admiration for how children can turn something heavy into something light. The poem leaves us with that small, hopeful image.