Dinner and belly buttons

I just love to cook, and it especially is fun when I have people over to share my meals with, so tonight we had a few friends over for dinner. We have done dinners at each others homes a few times now, and I love it. Combined we have 6 kids all under 5, so the kids I think have a good time, it gets a little crazy, but overall is enjoyable. The only issue I have is that my son likes to check out my friends daughters belly button all the time, and she doesn't like it as well, and it is more annoying than anything. So I was wondering if anyone else's kids have infactuations with body parts, or belly buttons? What do you do? Will he grow out of it? I know he is just a kid, but just wondering if there is something I should be doing different.