Is it possible that the kids are somehow misusing the shower faucet causing the cartridge to fail?
I would be looking at the valve plates to see if there is scoring from the original failure. If some damage was on the plate then a new set of seals may work at first but fail as the scoring wears them down. In addition if you find rust scale fine rocks or lime build up in your airators on your faucets it could have been a chunk that was still in the valve when you replaced the last one, I usually try to flush out the plumbing when repairing a washerless for this reason. So could it be the kids sure but I doubt it.