There is no way of policing this but I kind of agree with the principle OP - its more about consideration for the needs of others and social responsibility, (like not panic buying or hoarding, or taking a holiday to a rural community now)
We've been in lockdown for 11 days now (Denmark) but are allowed to go out/take kids out if you observe distancing stuff. There are some great playing fields near our house but we have a garden and there are a lot people living in flats nearby, who's main chance of fresh air while observing social distancing is those playing grounds. So I've said no to the DCs when they have asked to go there, because I think those stuck in the flats, with or without children, have more need for it than us at the moment. If those with alternatives avoid using the area then the density of people is reduced overall and those who dont have many other options get best access. DCs have constant access to the garden and we can go walking/cycling in other directions with less people so the playing grounds are off limits.