Sim child taken away for no reason

I have 2 sim children, who are both A students (not cheated, they worked and got promoted). I received notifications for both of them a while back saying 'Mathias is being neglected, improve his grades or social services will take him away' (something along those lines). I assumed it was just a glitch as both sims have been A students for months (I play this family very seriously and they have been in the child state for 4 human months). However, just now I got the death pop-up saying 'Mathias has passed away would you like to have a playable or roaming ghost?'. Out of panic I clicked playable, and a 'due to neglect Mathias is being taken away pop-up occurred, and then he disappeared. I exited the household without saving, but he now doesn't show up in the family when I click on them. The only mods I have installed are UI cheats, realistic childbirth mod and McCommand Centre (which has all the default settings)>

What did I do wrong and how can I get my child back? Also, how can I prevent his brother being taken away (who got the original pop-up shortly after), even though he has A grades?