Ask HN: Open-source/paid recommendations to run babysitting club

Didn't think I'd ever be posting this on HN, but here goes. When I was a kid, my parents used to share babysitting responsibilities with other parents in our class so they could go out every couple of weeks. I was hoping that there would be a tech solution to manage this but I'm struggling to find one. Basically we want an app/website to start a group, people can join and they're given X tokens to use for sitting which are transferred on a case by case basis. They can't buy new tokens, we don't want people to start using it as a paid service. Anyone have any ideas or recommendations? The ones I've taken a look at seem to be expensive and subscription based ($10/pp month + can pay for babysitting). We're all happy to pay a one-off fee or a reasonable annual subscription amount.