The SSA has a new word for our changing situations, GRANDFAMILIES
Children living in grandfamilies can be eligible for the child benefit through their parents or grandparents.
Some children in grandfamilies may qualify for grandchild benefits. Individual cases vary and families should contact the Social Security Administration to see if a particular child is eligible but generally the following conditions must apply:
• The biological parents of the child are deceased or disabled, or the grandchild must be legally adopted by the grandparent.
•The child is not receiving benefits from a parent when the grandparent retires, becomes disabled, or dies.
• The natural parent(s) of the child is not be making regular contributions to the child’s support.
• The child is unmarried and under 18 or up to 19 if attending school full time.
• The grandchild began living with grandparents before age 18 and received at least one half of support from the grandparent for the year before the month grandparents become entitled.
On the surface it appears your grandchild may not be eligible to apply under your SSI but you should seek a direct answer form SSA.