Not Dutch but Estonian here. I grew up playing outside, but I'm from a quiet street in a small town. In the cities, one of the actually positive influences of the soviet empire is that they built playgrounds between large apartment complexes so as far as I know, kids played outside unattended there too, but it would've likely been slightly older kids, not like a 5 year old being let out with his 8 year old friend (yes, talking about myself here).

Not sure how it is these days though. I see fewer kids running around on the street than when I was a kid. Suspect it has little to do with traffic or crime and more with the availability of entertainment via TV or phones. 25 years ago over here we had like 3 TV channels, for anything else you would've had to pay Viasat or someone - which a lot of families couldn't afford. Of course there are also just fewer kids in my hometown (at least on my street) and I have no comparison data for the bigger cities at the time of my childhood.