Re: Sims 4 added to child account, but EA app just shows a blank square

@Unsnoozual, great, good to hear that that helped!

Regarding the ineligibility message, speaking for the EA side at least, Child accounts are blocked from online features to protect the user and comply with laws and regulations of your region. 

Some of these restrictions are set until the child account ages up, even if you have made changes with parental controls. Due to this, the account would be blocked from linking to other accounts or social features. Is your older daughters account a teen account? That could explain, why one account worked, but the one for your younger daughter gave you that message.