8 would surely be a more practical number than 6 due to the numbers of 2 children families. Not that it helps 3 child familes, but 6 is more than two families of two parents and their average 1.9 children each. I suppose I could leave DS2 behind as being the smaller child, he makes up the 0.9 nicely 
Back in a pragmatic real world, you'd have to be a particularly Vogony dementor, with well-clutched pearls and very twisted knickers to object to a gathering of two families with children. The tone I get from Whitty and Valance at the briefings is that is the guide, they have to draw the line somewhere, and use your brains to deal with the small print.
I suppose that by the time you get to the teenagers hanging out stage, 6 from 6 househoĺds is plenty enough mixing.
The problem is you can't have too many caveats as you'd never cover everything and people can garble even the most basic of messages.
Anyway, I'm off for a picnic. Which will do more social harm, ham and pickle sandwiches or falafels?