Is it permissible to use money saved for the children to perform Hajj for the parents?

It is permissible for a father to revoke a gift he gave to his son as long as the son has not disposed of it. The father may revoke this gift because it has not been taken possession of. If the money is a bequest, it is permissible to revoke it because it is not binding until death. Therefore, there is no harm in using this money for Hajj for you and your wife.