Although TMJ / TMD (TMJ disorder/dysfunction) is commonly seen in adults, it can be present in children at an early age.
The common condition seen in children is called Bruxism, the medical term for the grinding of teeth or the clenching of jaws. Bruxism often occurs during deep sleep or while under stress. For instance, a child might worry about a test at school or a change in routine (a new sibling or a new teacher). Even arguing with parents and siblings can cause enough stress to prompt teeth grinding or jaw clenching.
Unfortunately, if undiagnosed and left untreated, the symptoms and condition increase with age. The good news with children is that they are still in developmental stage and the earlier the problem is diagnosed and treated the better the results!