Ninjago is basically the platonic ideal of a kids' action show—exciting enough to hold attention, positive enough that you don't feel guilty, and LEGO-based so it actually encourages creative play when they're done watching.
The ninja violence is ever-present but handled well: weapons flying, villains defeated, but zero blood and a generally silly tone that keeps it from feeling dark. Some moments (disintegrations, off-screen eating) might spook younger viewers, so save this for kids 6+.
What makes it work is the genuine emphasis on friendship and perseverance. The ninja lose, struggle, and have to work together to win—not a bad model for elementary schoolers navigating their own social worlds. Plus, with over a decade of episodes, if your kid latches on, you've got content for days.
It's not going to teach them math or science, but it'll keep them entertained while modeling decent values and maybe inspiring some epic LEGO builds. Solid choice for the action-loving crowd.