Here's my 2 cents: it's impossible to cheat as described. There is no way that two elementary school kids rated under 1200 can systematically help each other by talking. This is not because they are bad at chess, it's because they are terrible at describing things. You know how many times a day one of my students answers a question with "e5" when he means "Queen e5" or "take and then play Rd1" when there are 3 things you could take and either rook could move to d1?
OK, there might be certain very specific situations where one kid notices mate in one and the other doesn't, but anything short of that and the chances of helping are close to nil. Let's run through a couple scenarios:
1. The position is tactical and the teammate sees a winning line. They go to the bathroom and teammate tells player his variation.
Problem: The stronger kid will say something like "trade bishops and then play queen check." First of all, kids can't visualize, so there's a chance that the stronger player says it wrong, plus there's an excellent chance that whatever is said is ambigious, and add to that a reasonable chance that the player remembers it wrong. Even if the kid remembers it perfectly, if his opponent plays a response the teammates didn't discuss, good chances the player panics and blunders.
2. A kid sees a plan and tries to describe it.
Problem: Seriously now, a kid walking by who is in an under section is not capable of conceiving of or articulating a reasonable plan in the minute he/ she looks at the friend's board. And even if he could, the kid playing will not be able to understand what he's being told when he's in the bathroom and can't see the position.
Try this experiment. Set up a position where you understand the plan for one side, let's say this one:
Find an elementary school age kid rated 1200 or less and ask him to tell you what to do. I think you will find the results amusing.
(This is a very standard pawn structure-- it can come out of many openings in this form or reversed. White's plan is to play on the kingside, using the open e file and e5 square, i.e. Bg5, Ne5, R3e-h3 Q=> kingside. Black's plan is a minority attack, i.e. Rb8, b5-b4, Rfc8, Na5-c4, Q=> queenside. )
Just to be clear-- I'm not saying cheating is impossible. If I were hiding in the bathroom stall telling my players where to go and what their plan should be, I think I could be helpful. But this is because I know extremely well what my kids understand and what they will understand by what I say. Plus I'm good at chess. I also suspect my older and stronger students could help each otehr, but I'm talking about 13 years old and 1500.
Quite seriously, if I thought another team was cheating in this way against my students, it would not even cross my mind to complain. I would just laugh and add it to my list of Stupid Things Other People Do. There's a quote from Kramnik, "If you think your opponent is going to play the Dutch, don't do anything to discourage him." That's how I feel about this.