Couldn’t find good stats for shootings and children but it looks like 1000 kids die each year in the US from car accidents so all it would take is 10 kids dying a year in shootings which doesn’t seem like many at all (just consider kids in street gangs etc).

‘Mass shooting’ has different definitions depending on country/agency. In the US I think it just means a shooting (not necessarily a death) involving more than 2 people.