10 days off school/sports/other activities when they get it.
... and that's if it's symptomatic. If it's not - or if their parents send them to school anyway - they are likely going around distributing the generally-harmless-to-them virus all over the place
, placing any number of vulnerable people of all ages at more risk than they need to be.
Mind you, how many non-masking adults are doing the same?
That is the reason we should still be masking when in confined indoor spaces with strangers - any one of us might be asymptomatically infectious, and able to infect the person next to us in the bus or crowd or queue who, unbeknownst to anyone, is one of that 10% who didn't respond to the vaccine or is otherwise vulnerable, yet who appears, currently, to be perfectly well.