Is it permissible for a person to give his Zakat to his orphaned grandchildren, and is Zakat permissible for them, knowing that the grandfather is well-off?

The maintenance of poor, non-adult grandchildren is obligatory upon their grandfather from his private wealth. It is not permissible for him to give them from zakat, nor can zakat from others be given to them if their grandfather provides for their needs sufficiently. However, if the grandfather refuses to spend on them, or what he spends is insufficient, then it is permissible for them to take from zakat to supplement their needs.