Is the father entitled to reclaim a property he bought for his minor children in the name of their deceased mothers, especially since the contract stated that it was a sale contract and not a donation?

The principle in contracts is based on meanings, not on mere wordings. If the property was a gift from the father to you without any compensation, and the writing of the sale contract was merely a formality, then the father has the right to revoke his gift after its possession, unless the property has undergone a change by the children or a third party's right has become attached to it. This is based on the saying of the Prophet, peace and blessings be upon him: "It is not lawful for a man to give a gift and then take it back, except for a father regarding what he gives to his child."