Are Tokyo and Osaka notably sparse compared to American cities?
Maybe I was in the wrong parts of Tokyo/Osaka, but I do try to walk all over the place when I travel; there weren't a lot of families going out to eat, or at the malls/shopping areas, or whatever.
However, you'll see train cars full of young kids going to and from school, unsupervised as far as I could tell. There are parks with play equipment all over, and I would run into kids during the day at combinis or 7/11s or whatever being kids. Probably more like what I remember growing up in the 70s was like in the US. I can't imagine a couple dozen third-fourth-fifth graders jumping on a train unsupervised in the US.
A lot more children out riding bikes unsupervised, as well.
I was in Tokyo for the golden week last year; there were lots of kids and parents running around. It might be that Tokyo is an internal tourist destination for that week that skewed my perception, though.
It's rather the opposite - families leave Tokyo to visit the grandparents still living in more rural areas. But when schools are out you'll always see more kids.