I always tell kids what my mom told me and my sister when we were little, that holding kitties too much will make them sick. Yes they are cute and they are tempting, but just like human babies, they need to be left alone for their mom to take care of them.
They need to eat and sleep and be left alone, especially when they're that young.
Kids usually understand that.
Also, you just need to remember the magic word. NO.
Just tell them no, not today. Maybe after school on Monday. (Or whatever day you think).
What are they going to do, go home and tattle on you? You have every right to say it's not a good time. And, since you're friends with the parents, I'm sure they will understand that. Tell your friends that you have to keep your own kids from messing with them too much and they make the mama cat really nervous. That way you're including your own kids so they can't take it personally.
Really, they shouldn't. Just a peek every now and then is all anybody needs. They are way too little to be handled much.
It's been a week, time to slow it down now. My cat had kittens and I didn't let anybody hold them. I checked them out to make sure they were okay, but that was it. I let my kids touch their little heads and we patted mama to tell her what a good girl she was, but we left them alone. She was such a great mother, she knew what to do.
Best wishes.