The law for this is outdated, when not all of the children of the deceased live with the ex-spouse. In essence, I feel as though the government is saying it is more expensive for the ex-spouse to raise the children in his or her home than it is for the other mother's and father's of the children born out of wed-lock.
If you are caring for a child under age 16 or disabled who is getting benefits on the record of your former spouse, you would not have to meet the length-of-marriage rule. The child must be your former spouse's natural or legally adopted child.
However, if you qualify because you have the worker's child in your care, your benefit will affect the amount of the benefits of others on the worker's record.