U11 regulations
8.e) A “ruck” is a phase a play where one or more players from each team, who are on
their feet, in physical contact, close around the ball on the ground. Open play has
ended. Players are rucking when they are in a ruck and using their feet to try to
win or keep possession of the ball, without being guilty of foul play. No more
than three players from either team (including the ball carrier and opponent)
can be involved in the ruck.
So if you have a as described "the kids are all diving in to the ruck trying to grab the ball and mostly going off their feet so we end with a big pile up" just restart with a scrum. Alternately the ref should shout "no more" or tell them to "stay out" once the maximum number in a ruck is achieved.