Free throw rules question

[pmasonf]

7th grade girls travel team, playing in the league playoffs. The other team has a fast break. Our player chases the girl with the ball, their feet get tangled and the other girl goes down hard. Our player is called for a foul (correctly) but the other player is injured and leaves the court. The ref lets the other coach bring in his best player (who happened to be on the bench) to take the 2 foul shots and she makes both. Immediately after, the "injured" girl comes back in. A few minutes later, one of my players is fouled hard and, being pretty savvy, tells the ref her wrist is hurt and she can't shoot the free throws. He tells her she's not hurt enough and makes her shoot. What should have happened in both cases? Fortunately, we won the game so it's more a case of wanting to understand than anything else.