This has been close to 2 hours of my time to sort this, should be much much more straight forward, hopefully the process for Sims 5 will be much improved and likely the reason it's such a ballache for Sims4 is cos more resources are going into dev of the new version....
In order to get The Sims 4 base game added to an underage or child account, you'll need to reach out to the live support team so they can manually add this content on the account for you- here on the forums we aren't able to access your account information to ...