You know, it really bugs me when parents don't do any research before taking their kids to the theatre. Not only do many shows contain material that's inappropriate or a long running time, but there are practical considerations as well. Most theatres just aren't equipped for kids. It's really difficult for a child to sit in a theatre seat because, frankly, they're just too big. Very few theatres have cry rooms or other places where restless children can be taken during the performance.
It's the same way at the children's theatre I work for. So many of our adults assume that all our shows are appropriate for all ages, when that's just not the case. One of the most effective things we do is offer age-appropriate subscriptions for parents of young children-- that way, we don't end up with five year olds at our more mature shows. But the burden needs to remain on parents to know what they're exposing their kids to.
Hopping off the soapbox now...
"I know what the sunlight can be"