A lot of them are/would be. If you look at a lot of inactive brickfilmers/older brickfilm channels (where the creator probably isn't as active and/or doesn't care) the majority of those brickfilms have been set to 'made for kids' already.
Similar to Adult Swim, or Family Guy, or Robot Chicken, though - just because brickfilms use a medium generally aimed at kids (construction toys, or cartoon animation and vintage kids toys in the aforementioned cases) doesn't mean the content is suitable for a child audience.
As Lech said, the creator sets the intended audience.
Although, I'm of the opinion that all eligible brickfilms should be set to 'made for kids,' since the new COPPA compliance isn't about signifying audience "hey, my videos are for ALL ages" but rather about preventing YouTube/Google/Others to track minors on the internet.
COPPA was passed in the early 2000s, and every website has had to comply with its guidelines. YouTube thought their kids app was enough, but clearly it wasn't. Loads of videos kids watch weren't on the app and a lot of parents and kids just didn't use it/know it existed and used regular old YouTube.
Now, one could make the argument that instead of forcing content creators to label select videos, YouTube could have made it mandatory to sign in before watching videos - thus filtering content viewed by kids' accounts, but this would have greatly lowered view counts and ad revenue for a lot of people - including themselves.
So they chose to put the responsibility on the uploaders. And boy is that not working out.
'Made for kids' videos can be viewed on either platform (reg YouTube or the kids app) though, so I set my own appropriate content to 'made for kids' so it can get exposure on both. A lot of the new changes & limitations suck, but I think many are overreacting to it and their refusal to comply could come around and bite them in the rear... however unlikely that may be.
But hey, even LEGO has made sets & references to non-children's content. (Stranger Things sets, Agents from the Matrix, and R rated film, are seen in the background of The LEGO Movie, etc.) So, you can see why there's debate and a some community members decided to jump ship to another hosting site.