Playdates
Hosting other people's children without losing the house.
Playdates go wrong in predictable ways — over the favourite toy, over whose house it is, and about twenty minutes before the end when everyone is tired.
These pieces cover the practical fixes: putting the most precious things away beforehand, having one activity ready that needs two people, and finishing while it is still going well.
Parallel play is fine. Two four-year-olds in the same room doing different things is a successful playdate, whatever it looks like.