I was playing a tournament and some kids were talking about the game in a foreign language I was playing and I am pretty sure that my opponent was talking about the game with his friend in another language. I don’t speak the language, but you can tell. What should I have done?
First, you should inform the tournament director, and let that person handle it. There is a parental instinct that makes me want to just take the kid aside, and say something privately first, but since that can be misconstrued, do not do that, and let the TD handle it. I am sure that they will mention it, or warn the kids, and hopefully it will be done with.

I speak Russian, and in times gone by, I would hear some of them discuss games that were not mine, and it was not so much about specific calculations, but still. Thankfully, it was not common, but it still violates the rules, and needs to be dealt with. In my case, they were always adults, who should have known better. In your case, you have kids, who often just have not been taught well enough yet, and hopefully a stern word of warning will end this before it is evolves into intentional cheating.

Playing kids is hard enough: To have to play two of them in tandem is for the birds. :)