Are the shares written by the deceased father in the names of his children—while he retained their ownership and possession during his lifetime—considered part of the inheritance, or do they remain the property of each child in their name, given the disparity in value between the children's shares?

The aforementioned shares are considered an inheritance and are to be divided among all heirs according to their prescribed shares, because a gift is not complete except by possession. If the father retained ownership of the shares, then this is not a gift but rather a bequest to an heir, which is invalid unless approved by the rest of the adult, mature heirs. The matter of estates must be referred to the Sharia (definition) courts.