Once upon a time I thought it was "cool" to pop my jaw, now, not so much.

I use a mouthpiece (just like American football) and massage the joint regularly. It took about 2 weeks to get any noticeable improvement but I sleep better now. If I miss a day, or a night, even a 1 hr nap without the mouthguard I feel it immediately. It also irritates my tinnitus.

I'm not sure if this helps with a 12 year old but maybe it's a simple start.

 

My daughter got a special mouthpiece from her dentist that positions the jaw to relieve the symptoms. It looks a lot like what you get if you grind your teeth.

 

Without knowing the full history, TMJ normally stands for the temporomandibular joint. There are a range if disorders that can occur within the joint. Best to seek specialist advice.