But then shouldn't DL have been notified in advance and seats assigned together? If sitting together is important and there's a disability reason for this, I find it hard to believe that they weren't aware before boarding that the seats were all separated.

Moreover, for now I'll stick with my hypothesis that either the autistic kid or someone in the party ended up in an exit row, which is not allowed and would indeed be a safety issue.

This not to say that it was right for the FA to just order everyone into their seats and take a fit.