Does the money bequeathed to the deceased woman’s grandchildren (a male and a female) all go to the female after the male dies during the lifetime of the testatrix, or does the male’s share revert to his children?

Your mother's bequest to her two grandchildren is permissible because they are not among the heirs, and the bequest does not exceed one-third, in accordance with the saying of the Prophet, peace and blessings be upon him: "Indeed, Allah Almighty has bestowed upon you, at the time of your death, a third of your wealth as an increase for you in your deeds."

In the event that the legatee dies before the testator, the bequest becomes void according to the majority of scholars. Therefore, your deceased nephew's share of the bequest becomes part of the estate and is divided among all the heirs.