I can't stand Jackie Hoffman on-stage -- she is a ham -- so that may impact my view. If she is going to accept roles in shows like The Addams Family and Chocolate Factory, what the hell does she expect? Add to that the fact that the quality of both shows is pretty poor -- that based on viewing Addams and reading the feedback here -- and kids minds are going to wander.
The key contributor is that parents who love the theatre and can afford the prices, can't wait to take their kids to the theatre so they can develop that love at an early age. They are not thinking clearly.
I did it when my kids were young, and it worked sometimes (e.g.,The Lion King, The Producers) and didn't work others (e.g., Phantom and The King and I). At the King and I, my (I guess a little spoiled) 5 year old son was on the aisle seat in the orchestra, when he decided to get up and practice his bowing in the aisle. When I realized what he was doing, based on laughs from behind us, I pulled him back into his seat, but thought 'boy, I guess he is really enjoying it'...only to hear him announce five minutes later to anyone within 5 rows of us to hear that he was bored...luckily, we were not that close to the stage. I concluded at that point that further visits would be restricted to Disney and Peter Pan for several years.
I do think that parents could do a better job letting their kids know that they need to be quiet before the show begins. Based on my experience, some have not.