FGS, the OP isn’t talking about going in all guns blazing and creating a scene, she’s talking about asking politely whether the seating plan could be such that her DC with SN sit with their family.
TBH, if they’re literally planning to seat no-one together who knows each other I would also be asking whether they thought that was a good idea as people won’t necessarily talk to one another. It’s one thing to have a couple of couples who don’t necessarily know each other at a table, quite another to literally separate everyone
And she’s going to be at the top table, with people she wants to be with, so what does she really care where anyone else sits?
Besides which, the seating plan isn’t usually done until a few weeks before the wedding. Here the invites haven’t even all gone out, so she can’t actually even think about a seating plan.
FWIW when I got married we had the idea of seating different people together so it wouldn’t be too cliquey. Not individuals but couples iyswim. My FIL said it was a bad idea and that if we did they would suggest their friends move everything around anyway. As I wasn’t going to be sitting with any of them I just smiled and nodded and we put people together who knew each other. No big deal.
Seriously it’s the dress, the service, the food, the friends and family which make the day. Not the bloody seating plan.