To link the two accounts you both need to be using the same email address.
For example if you're email address is 123@123.com, his has to be entered as that on his account as well. Thats what links them together. Once thats done, you'll see him in the Manage Kids area on your own account
Last week my 13-yr.-old nephew and I established separate chesskid.com accounts. I purchased a gold membership intended for him, but it accrued automatically to my account and there appears to be no way of transfering it. I contacted Member Support and asked how to do this. They told me I would be able to use the "Manage/Add Kids" to do this "if [I] have his account linked to [mine]." But I don't, and I can't see how to link it. The chesskids.com interface is primitive and there's no member support (that's why I'm posting in this forum). There's also no way to communicate directly with chess.com member support, only by email and their replies are always inadequate. Can someone please tell me how to link two chesskids.com accounts to one another? Thanks.