My sense is that your husband's former wife and the mother of the child they had together received a communication from SSA advising her about the child's eligibility for dependents' benefits on his father's account. I strongly recommend that you contact your SSA district office to make certain that there are no further inappropriate disclosures of information about other dependents.
I would be concerned that SSA may not have the correct address for your husband. You need to meet with an SSA staff person about this matter. Good luck with this.
This advice is not a substitute for a thorough evaluation of all of the facts of the situation with an experienced SSA benefits attorney.