Is the OP asking whether parents should be able to lock kids up in their rooms? If a young child is locked in a room one may break down the door on shabbos as it is potential pikuach nefesh.
Certainly one must not l’chatchila lock a child in a room – unless the child is older when there is no longer a sakanoh. Thus the shaylah is at what age is it no longer a sakonah to lock up a child in a room?
The OP is assuming that it is legal and not against dina d’malchusa to do so, which is in itself a shaylah that requires legal expertise.