On our last trip, we signed up DS, DD, and one of the cousins, and they loved it! It was a lot of fun for them, and we got some really great pics.
The only downside was the long line to sign up - we got there early, and a cast member directed us in the wrong direction (I think she misheard us, actually), so we got stuck and were not there as soon as signup began. Still, even though we lost the better part of an hour, and it did tie us into being at certain places at certain times (since you meet up before the actual event), it was probably one of the highlights of the trip.