There is no SSI computation that involves an income limit and dependents for SSI payments. There can be an allocation given to children when determining the amount of deemed income from a spouse. So if your spouse had income from a job, pension, unemployment, whatever; and some of that money is used in your computation, then there is an allocation for each of the minor children (mother or father relationship) without income that also live in the house. But a grandchild would not count unless adopted.
So you question as it stands, answer is no. But neither would any other child living with you count as a dependent for deciding an income limit for SSI. How many people you support has zero effect on your SSI payments. No matter the relationship.