How typical is diverticulitis in kids?

Never seen it: I've been in pediatrics now for 3 decades and never seen or heard of a case of diverticulitis.

Colonic: Diverticulitis is very rare as diverticulosis(the out-pouching that become inflamed in diverticulitis) is an age dependent process. It takes times for these issues to form, usually decades. So unless a child has a predisposing condition like Marfan's syndrome or cystic fibrosis, it would be exceptionally rare to see.