Is it permissible for the maternal grandfather to pay for his grandchildren's education expenses from his zakat money if their father refuses to pay them, and is the grandfather obligated to spend on them?

If the children are able to earn a living, then their father is not obligated to provide for their education, unless earning prevents them from studying an individual religious obligation (fard ayn) or communal obligations (fard kifayat) and there is hope that they will benefit Muslims; in that case, their father is obligated to provide for them. If the father refuses to provide for them and the children are unable to compel him to do so, then there is no harm in them taking zakat from their maternal grandfather or others, because in this situation they are considered not self-sufficient.