Ibn Qudama says in his book Al Mughni–
Ahmad bin Hanbal said: It is better if he doesn’t distribute his wealth and he leaves it as one of the obligations to Allah SWA. Perhaps he may have a new child, and if he had given his child all the wealth, then he begets, Imam Ahmad said: I would prefer he gets all the wealth back and make it equal between them. Meaning he has to go back to each of his children, and returned all he had given all of them so that they may pay him to give to the newest child, so that he is equal with his siblings.
And if this new child is born after the death of his father, then there is no need to go back to his siblings, as giving was conditioned on the death of his father, apart from another narration, which Abu Abdullah bin Battah has chosen. There is no difference in opinion that it is desirable that whoever is given a share should consider giving an equal share to his brother, and for this reason Abu Bakr and Umar RA, ordered Qays bin Saad return his share of wealth, in order to include in the distributions the newborn baby, and have with them equal share, after the death of his father.