Cave Story is an amazing game and one of the best on 3DS. The music, graphics and gameplay is some of the best ever made, and considering it was made by one guy is inspiration to create something truly wonderful.
This review makes me think you didn't play the game. Because this game is actually quite violent. The plot follows Quote and Curly Brace, who attempt to protect these bunnies called Mimiga from harm. Although the game does become incredibly dark. For one thing, depending on your choices in the game, Curly Brace can die a horrible death after she gives you her air tank so you don't drown. Also, the Evil Doctor who is kidnapping Mimiga, force feeds them red flowers, which as described by one of the Mimiga: "Causes their blood pressure to skyrocket as they drop dead." Not to mention that one of the Mimiga, Arthur, is killed while trying to save the Mimiga from harm.
With that said, don't let that keep you from playing this fantastic game. It's not for youngsters, but if you think your kid can handle it, you're giving them one of the best video games ever made.