All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
A father is entitled to take back a gift that he had given to his children especially if it [the gift] was not just (gifted equally) among all his male and female children; for more benefit on being just when gifting the children, please refer to Fataawa 94685 and 96941.
However, the father can take back the gift only if the son did not dispossess the gift by sale and the like.
This is, of course, if we presume that what the children have is a gift and consider that writing the property on their names is a valid gift if it is proven that they acted about it (the property) in the same way an owner disposes of his ownership.
On the other hand, if the father had written the property on the names of his two sons but he acted about it as its owner, then this is not a legal gift and no consideration should be given to the law which gives the property to the children by that written document.
As regards the work of the two sons with their father in a project which he owned, then this does not give them the right to own that property. Rather, what should be looked at is if they entered with him in the project as hired workers, then in this case they deserve the fee of people who do the same job like them. But if they entered the project as volunteers, then he does not owe them anything. If there was no agreement between them, then the customs and traditions should be referred to.
In any case, if the matter is taken to an Islamic court, then it is the Islamic court that should rule about the case. However, if the case is taken to a court that rules with man-made laws, then they should withdraw the case and take it to an Islamic court, or appoint trustworthy scholars who are knowledgeable about such matters to rule between them.
By all means, it is not permissible for the children to ill-treat their father by defaming him or insulting him and so on, as this is considered among cutting ties with kinship. Indeed, Allaah ordered us to treat kindly a non-Muslim parent, let alone a Muslim one.
For more benefit, please refer to Fatwa 82254.
Allaah Knows best.