I have family and friends with their kids come visit very occasionally, but it is never unannounced. I always try to make the visits with my friends with kids be outside, as I find the visiting kids tear my house apart in a way that the daycare kids never do. I always find it amazing how 2 of my friend's kids can make more mess than 6 daycare kids.

But I don't mind the visits (which aren't very often), even with the visiting kids not knowing the daycare rules, there usually isn't too much distruption and the kids all mostly play well. If this wasn't the case, I wouldn't ever have visitors.

As it is never unannounced, I let the daycare parents know that I have visitors coming.