Child guardianship , social security benefits

My daughter who is now deceased and was married at the time, was never married to her two children's father. The children ages are 11 & 14. I am the father of the deceased and has had the 14 year living with me the last 10 years. The daughter always lived with the mom. How do I go about making sure the surviving children's benefits go directly to the children and not the father, who has never had custody of either of the children