-Not the OP, but living in a Norwegian small town (population 5,000), my two oldest kids (aged 5 and 10) are just sent outside to find someone to play with - they have several friends living within a few minutes' walk, and what sparse traffic there is is moving at less than 20mph.
While there sometimes is some parental organisation by phone or text, for the most part, local kids just have to go out to see who's available, then be home by whatever time we decide to be the latest.
I realise this will not work across all locations, but out here in the boonies it works a charm.
If some kid comes by asking for my kids and they are not in, (s)he just walks to the next house or playground trying their luck. If I expect my kids back soon, I just invite the kid at the door in to wait for them.