Mama called this "messing and glomming."
Looking at the poll, most must agree with Mama.
*Update!* Judging from a few of the comments,
.....Sorry...I agree with it. There is no reason for that much wastefulness
.....I'm sorry but I have to agree with the restaurant
I need to clarify myself. I agree with Mama. And her phrase, messing and glomming, means to waste or leave in such state as no one else can use it. And the poll reflects that most of the people who responded to it DO agree with the restaurant.
Kids will be kids, anyone can over-help their plate. And yes, sometimes at a buffet you are stuck with the burnt edge of the pan and might want to upgrade to a better helping when a new pan comes out. But normal over-helping wastes only a few bites. And a parent who knows kids will be kids, will direct them when helping their plate to put only a few bites until the have tried it. In general, it is always better to go back for seconds than to overfill the first time around.