Is it permissible to contravene the deceased's will not to divide his inheritance, for the purpose of his children's reunion, if some of the heirs desire their share?

This will has no legal standing in Islamic law, and the heirs are not obligated to implement it. Anyone entitled to a share has the right to claim it because the matter of inheritance belongs solely to Allah. He, the Almighty, has divided it Himself and given everyone their due right. The heir acquires ownership of the inheritance automatically upon the death of the deceased, each according to their share as determined in the Book of Allah. Whenever one of the heirs requests their right, the inheritance must be divided, and everyone must be given their due share.