
There is no "required" information. Full names would help plus a statement that the are his grandchildren or your children. He must amend his previous will with a document called a codicil or make a new will entirely. I will observe that your father is not required to do anything and it might be diplomatic to co-operate with him..
Other Answers
Your dad can make a gift to the children of [your name], without naming your children's names.
"I give the sum of $10,000 to each living child of my daughter, [your name]."
He can either do a new will, or a codicil to his existing will.
He could also make a gift of money or property to your children as a class, listing their names, or to each one of them individually, specifying their names. It is also common to specify the relationship:
"I give the sum of $10,000 to the children of my daughter, [your name], in equal shares."
"I give the sum of $2,000 to Jane Doe, my granddaughter. I give the sum of $2,000 to John Doe, my grandson."
Social security numbers are not needed to make a gift to grandchildren in a will or codicil, or in a trust or trust amendment. Social security numbers should never be listed in will, codicil, trust, or trust amendment.
I hope that helps!