Looks like a 9v9 game, yeah? U11 or so? Obviously the referee is in the wrong here, but I can 100% guarantee that if the game is tied or Legends are down big, that foul doesn't get called. That's the age range where you get kids scoring from nearly the center of the field off free kicks, just because they can air it over a keeper that mistimes their jump. The ref doesn't want to give away the free kick right outside the box (I'm guessing from the location of the other line judge), and that's why it isn't called.
The coaching lesson here-- not necessarily to the CB in question-- is to the other Legends fullback. Don't run with your teammate. If your CB is going to win the 50-50 ball, make space for them to play-- that should have been a covering run straight backwards. By running with them, you incentivize the "I'll use my body to just block out the forward while you take the ball" play, and that's where the lack of physical coordination from a 10-year-old kid winds up with situations like this. Guarantee the CB didn't mean to put your kid on the ground-- you can see her under-compensate on the collision and that's why she winds up on the ground herself.