If you can't separate the kids from the adults then maybe you can have some kind of "good players" section where you have to be at a certain skill level to even join.
Even still, as sad as it is, some kids actually get pretty good and it wouldn't elliminate all of them.
Oh and one last thing, can we please not be put into matches that are about to end? That's really annoying.
I think this is a great idea that could easily be implemented into any shooter. If the game has less than 1/3 or 1/4 remaining, it's impossible to rejoin. Usually the team with little players left because they were being dominated so it's not like you can usually come back for a win.