The benefits handbook explains,
Quote Quoting Section 213 - The "Child in Care" Requirement
If you are the wife/husband or widow(er) of a covered worker, you may be eligible for benefits at any age if you have a child of the worker in your care. To meet this requirement, the child must be entitled on the same earnings record as the one on which you are making your claim. The child must be under age 16 or disabled. If the child is disabled and over 16, you can be eligible if you are performing personal services for the disabled child.

Please note that the child may continue to receive benefits until age eighteen, but your entitlement will end when the child turns 16 unless he is disabled.

The child does not have to be physically in your custody to meet the "in care" requirement. If you are exercising parental control and responsibility over the upbringing of the child, you may still satisfy the requirement. For instance, if the child is away at boarding school, you may still be eligible if you are reviewing his work and providing parental guidance. Note however, that if you are separated from the other parent of the child you cannot meet the "child in care" requirement while the child is in the custody of the other parent.

A child is considered in your care if he is with you at least one day of the month. If, for instance, the child goes to the other parent on July 15th, and comes back to you on August 15th, you will meet the requirement for both July and August. If the child did not come back until September 15th, however, you would not meet the requirement for the month of August. For any month in which the child is not in your care, you are not entitled to benefits and they are suspended. Once the child returns to your care, the benefits may resume.

If the child is over age 16, but is disabled, you may still meet the "in care" requirement if you are performing personal services for the child. Personal services include such things as helping the child wash himself, feeding the child, dressing the child, and so forth. Personal services require that you render more services than for a non-disabled child.
What care are you actually providing, if any?