We've also see the standard disclaimer on all of ADRs and have never been split up (group of 3 usually, occassionally a group of 4). No kids in our bunch, but they still don't make someone eat by themselves.
On a side note: the only place where this might be an issue - and it's not that the party is actually split up - but at Sci Fi diner if you are seated in one of the cars, it's a little squished to get three adults into one bench, so someone might end up in front of or behind the other two. Groups of four are split into two groups of two, but you're still seated at the same car so you just have to turn around to talk with the rest of your party.