Spinkle, I am doing the same thing. We are having our nephews and nieces, and a few cousins' children (they are teens and will be well-behaved), but not the children of friends or colleagues. I am hoping that if their names are not on the invite they will know their kids arent invited. Hopefully. Perhaps we might say something verbally to the parents who we think might presume their kids are invited.
I dont have any nieces or nephews yet (first one to get hitched), but one of my cousins has kids, my MOH has a kid, my uncle has just had a kid with his younger bride, and very close family friends have kids, all invited, very happy to have them there. PROBLEM: my other cousin isnt married, still lives at home but has kids with his partner, and they dont live together.
I've tried to have this discussion with my mum and dad (its dad's side of the family), and its still ongoing. We've never met the 'partner' or the kids. They couldnt make it to the engagement because Cuz was working. Invites to the wedding will go out in February, so we're hoping to have a clearer view on things by then. We might meet them over christmas, but i doubt it. I dont even think our grandparents have met the kids...
So my theory is, Cus and partner Yes. Kids we've never met, No.