There is strong evidence that children eight and under need direct assistance with brushing teeth. The “Assistance Age” has been tweaked a time or two, but eight is the minimum age a child can brush on their own.
That recommendation is based on coordination development, time awareness, and concept understanding. None of those factors pertain to a lack of trying. It just takes time to be able to brush your teeth thoroughly twice a day for two minutes! Oh yeah, practice makes perfect, too!
There are several “combinations” of child-parent brushing sequences that have been tried over the years. We all should be brushing twice a day. Sometimes a child will brush morning and a parent will brush in the evening.
Sometimes it is a 50/50 proposition every time brushing occurs. In this approach sometimes parents go first then the child. The reverse can happen with child first, then parents. It all boils down to what works best in your situation.
However you slice it, parents brushing their child’s teeth at least until age 8 is an important part of establishing and maintaining oral health.