I think it's hard to get a read on this and it really depends. For example, where I live there's an elementary school in the neighborhood, low traffic (even though it is the suburbs), and there's a walking path and a playground at the school. When the weather is halfway decent, there are kids streaming through there all day. During the pandemic it was just a party back there, in a good way.
But not all suburbs are like this, and I do think that low density and focusing on cars over people makes kids unlikely to go outside. Where I live, yea the school is nice, but I've seen kids standing in the middle of 5 lanes of traffic (they are standing in the turning lane) trying to pass and standing there with, say, a skateboard.
This is definitely a problem.
On the other hand, I think kids are just staying inside and playing with electronics more too. But why are they doing that? Is the neighborhood not stimulating enough? Is there anything to do outside? Again in the suburbs I'm in, what is a kid to do? You can't really walk to any store, park (besides the school area), or anywhere else. You can't even bike anywhere. You'd definitely get hit by a car at some point.
So I think it's a bit of a cultural shift, sure, but I think that doesn't account for poor urban planning. Most likely in the past kids were just so damn bored that they'd overcome crappy neighborhoods because there was literally nothing else to do. Now? Why risk riding down a country road on your bike and getting hit by a car when you can just sit at home and text your friends?