First, a parent can't teach his/her child much. Most times his/her knowledge is not enough to help his/her child in the best possible way. Even when knowledge is enough, the child may not listen to the parent, while most children tend to listen to their coaches or teachers.
Second, a child can't always play against his/her parent. The exposure is too narrow. They may always make the same moves, use the same openings, and follow the same plans. It will be boring eventually and not helpful.
Third, a knowledgeable parent may think that s/he is good enough to criticize his/her child's games. You can always hear some parents berating their children in the tournament halls. The children will either retreat into a shell and ignore all their parents' speech, or be completely crashed second time (first time over the board) and quit the game forever. This is the worst part.