Here's my thing: I consider 95% of all PG-13 movies to be kids movies. And that's especially true for big blockbusters like DC and Marvel. They get nominated at the Kids Choice Awards every year, they have kiddie concepts attached to them, there's Happy Meal tie-ins, they sell their toys in Target next to the Hot Wheels, they sell lunchboxes and backpacks with those characters, commercials for these movies air on Nickelodeon or Cartoon Network. Those check all the boxes for kids movies. Yes, that includes The Batman or Doctor Strange 2 or whatever else people think is "too much" for kids.
This also applies to Transformers, to Barbie, to Avatar, and so on. There are some exceptions in the PG-13 blockbuster landscape of course. I don't really believe this qualifies for Jason Bourne or Mission: Impossible, since they have concepts and stories a bit too complex for kids and don't see tons of ads on kids channels. But considering Marvel movies have a big kid audience, are advertised towards kids, and have merchandise aimed at kids...yeah, I think they are kids movies. And that's fine. There's nothing wrong with enjoying something aimed at children. I have a fondness for all kinds of kids movies, including PG-13 fare.
And really, I use "silly kids movie" as a term in order to make people realize "Oh wow. I'm taking these arguments and fights way too far. This is something I need to curb back". How effective it is can be up for debate, but I think sometimes it gives people perspective that we need to chill on the fights and battles. Like what are we doing here? And frankly, "silly kids movie" is one of the softest digs you can give towards these movies IMO. I could say a lot meaner stuff about them than that and I know our users are mature and can handle something so innocuous being said about the movies they like.