Anecdotally I've lived in a few different places, and it seems to vary a lot, even by neighborhood. In Utah there are tons of kids around. In Alaska there were neighborhoods here and there with lots of kids, and many others with practically no kids. I would guess that perception of kids varies a lot based on which neighborhood you move to, but yes overall it is declining.
More kids are in neighborhoods with... more kids!
TH = townhouse
SFH = single family home
... I think
And for the Brits, that would be detached (=SFH) and terraced (=TH).
Though in the US, we have lots of newer terraced/town-homes than I think you have across the pond (where they tend to be older homes in industrial cities). And we don't really do semi-detached (duplex in the US).
Is that actually true, beyond this anecdote? FWIW, my own suburban neighborhood (a cluster of about 60 townhomes) has enough kids that there's usually a small basketball game going on in the parking lot. And often bikes left scattered around (until dinner time, when the kids pick them up and go home).
In my experience as a parent (my son moved out last year), there were always kids around, but they were scheduled down to the minute in sports, STEM enrichment activities, scouts, or church. And those kids from families that didn't have that kind of budget were glued to TV screens or computer monitors.
We tried to give our son free time to play, but there was nobody around to play with. The kids were there, just not available.
Now that we've moved to a smaller house, with THs instead of SFHs, it appears to be slightly better. More play in the parking lot. But, the kids still seem pretty tightly scheduled (almost all are in sports leagues year-round).