Ref: You Make the Call

A U13 match is about 10 minutes into the first half. You've noticed two opposing midfielders leaning heavily into one another while positioning themselves off the ball. It has now happened again, but this time while they were battling for possession. No pulling or tugging, but an increasingly aggressive play with their bodies. Is there a call, a conversation or do you let it go?

This really si a situation taht a referee would really to see in person to amke a really good answer.
There is no infringement of the LOTG for "playing physically", as this is also known as playing soccer,unless it is reckless, dangerous, excessive or are specific actions (tripping for example) to prevent the opponant from playing the ball fairly (offside is one fo these, as is impeding(aka obstruction). The specific infringements are listed in the Laws of the Game.
However, if the referee does observe the players "leaning on each other" off of the ball and behind play, the next stoppage would be the time to give a "knock it off" warning, as teh next time will result in a card. It will also show them that their silliness is being seen.
Rm,
I really miss my yellow and red card emoticons from DMN forum.