(Opinion bit)

There should definitely be an exception for federated users. @sag@lemm.ee did not sign up to lemmy.world and therefore did not agree to the ToS.

(I am not a lawyer, anyone else can correct the stuff I say below)

Since lw isn’t storing sag’s data (apart from public posts and comments) there shouldn’t be any concerns with child data protection. lemm.ee would be serving them content that under 18s shouldn’t view, not lw (unless they are hosting it, which I don’t think you do?). I may be missing something (again, not a lawyer) but what is the point of this other than being (in my opinion, a bit too) careful with the law?