We back onto a park, though no direct access through our fence.
It's a nice, very well maintained, well used and policed park. It's gates are locked at dusk to discourage anti-social behaviour. There are some bars nearby and there'd be a risk of people just sitting out drinking in the park in the late evening, or walking through noisily on their way home otherwise. I'd have thought teens might go in there regardless but I'm not aware of that happening.
The good points are that we don't have to cross any roads to get into the park, which will be great for dd going to play by herself when a bit older.
Also, we're not overlooked at the back and look out onto some lovely big trees, very relaxation-inducing. We're overlooked sideways into the garden by neighbours but we're not staring into anyone else's back windows and we never will be, as the park will be protected against development. No noise or unsightly gardens or rubbish from 'backed onto' neighbours either.
We do get all sorts of council-run events going on in there, not in the area immediately behind us but near enough to hear clearly, all daytime or early evening, up to about 10pm latest: fireworks, concerts, a fairground.
The distance between us and these events - our garden is dunno, 20-25 metres long, then event areas are probably 100m away either side - makes it ok. If they were right behind us, I'm not sure we'd get dd to sleep at bedtime those nights.
On sunny days there are people everywhere but that's quite a nice noise and the fence means they're not peering in.
I'd check the council's events listings, check if there's an active 'friends of' group, have a good look around at different times of day, ask local estate agents, shop-keepers and the council about how the park is used, look up crime stats by area and ask about them - hard to interpret, you may see positive but not very relevant results but lots of the same thing would tell you something.