The “maybe not for kids” is really only in reference to the complexity (such as it is) of the conflict system. My particular nine year old can wrap his head around it, but your average seven year old probably can’t, at least not enjoyably.
Tonally, the Larkspur Academy setting is completely kid appropriate, intended to work like the cartoons do, for the most part. In the rules in that setting, it is impossible to cause harm to a student/trainer, and the world is built to allow the “suspiciously unsupervised and naive kids explore the world” thing.
The other settings are a bit more “adult”, but not in a “grim dark” way. You could run a kids game in either of them, but the stakes are a bit higher.