Two cd keys registred on the same origin account

Hi I have bought two cd keys for both of my kids to play sims 4 at the same time. Since they do not have the skills to manage and download games i used my own origin account to register the first game. Then I bought a cd-key for my second kid, and I registred that on the same account. My problem is now that only the one game shows on my account, and if both of my kids try to play simultaneously one of them get a message saying that they can't play online. I tried to register the second game on a new account, but it said that the game was already registered. I hope there is a way to fix this issue. 

Best regards 

Bjarke Annesen