First- rest that wrist for a long period. If the pain comes back at all then STOP playing. It will only get worse if you continue to play with a sore wrist.
I just read Number1Coach reply. I agree. Plus I would also really take a good look at technique. Something simple may be the issue. Start from square one with a different teaching pro to look at her technique. Main thing is to rest the injured part longer than you think it needs rested. If you rush it then you are just re-injuring the site. It may take months to get it to stop hurting. See a sports trainer to get some exercises to help strengthen that area , all areas actually.
I had a wrist injury and it took a year to recover from it. Wrist splint at night. Finally it stopped hurting. I tried playing once with the pain and I could not hold the racquet tight enough on certain shots it hurt so badly, because I kept trying to play over and over again when it hurt.
Good luck to your child with getting rid of the pain.