K.B.
unspoken rules: We take turns. Host my kid one day, and I try to host yours next time.
We trust each other to make good parenting choices. If you show up for a play date with a list of things they can't do and special snacks to share, it's like you're trying to host at my house. I'll decide how it's going to be here, you decide at your house. I will ask how someone feels about TV or video games, but beyond that let me be the parent now.
We don't report every little last thing. It's great to report what was eaten, and overall what kind of stuff they did, but no one needs a play-by-play. A simple "they had some trouble sharing but things worked out great in the end" should suffice.
But bigger problems should be shared. See something odd or sense a bigger issue between the kiddos? Either try to talk at pick up, or schedule a time to delve into it deeper.