""Drinking alcohol in the playground is typically not breaking any law. ""
It is where I live in Liverpool and you do get arrested for anti social behaviour, causing a breech of the peace etc.
We have alcohol by-laws, that are enforced because of previous escalating trouble, the Police use discretion they on the spot fine for drug use, which doesn't seem to happen according to many on here, where they live.
There are alcohol licences issued on certain days, but play parks, around ponds etc are restricted areas.
The advice on here, that says it's fine to drink in a swing park, especially to someone not from the UK, is wrong.
OP next time phone your local Police number or 101, depending on where you live (according to MN) they will respond.
It's rough where I live, in a lot of extreme ways, but parents don't put up with shit like this, there would be local protest until the issue is responded to.