Wow, I would never expect that. I mean, I would be happy to invite one more child over, but what if they have 3 siblings? So now instead of 3 kids, I have 6 kids over at my house? Of different ages? And will they all play together, or will some get bored and start trashing my house?
That may sound harsh, but it's happened. At this point of our lives, I pretty much invite MY friends over, and they bring all their kids. I had to stop doing that with one friend because the two oldest weren't as interested in playing with my kids, so they'd go off on their own. They trashed my son's room, getting out all of his toys, unmaking his bed, and they even broke a lampshade. And one of them kept sneaking out into the back yard and dumped the cat food into the water dish, making a huge mess and wasting our cat's food. I love my friend, and her kids are really great, but we just couldn't keep track of all of them in a new environment. And that was with TWO parents there!
As friends, though, we wanted to get together. We found that going to her house worked better. Really, I was adding one kid to her 4, because the baby stayed with me. And other times, we met at a park with all our kids. So maybe if someone insists that all the kids come over, suggest that you could all go to the park together (including the mom!)
My mom always said that if you need to get rid of all your kids at once, you have to pawn them each off on a different friend. Spread out the fun and the workload.
So don't feel bad. Don't send both your kids over if they're not both specifically invited, and make sure to specifically invite just the one friend. Be a force of change!!!