I grew up in a community where he had demolition derby's. It was a community event, and safety was always present and apparent. Also as a child I knew the dangers of doing this outside of the arena.
I do not find the violence to be bad at all. No one is getting "hurt", only their boats are. I remember crashing my Hot Wheels together and making them do flips, and playing this feels more like that than inflicting violence on anyone.
Talk to your kids about the encouraged crashing of vehicles and this shouldn't be a game you have to keep away from kids.
This game is great, really good fun for little kids, that will have a blast. Won't even cost a thing to get the game if you did what i did, I signed up at this link gifts4points.com/home?ref=amirshaw and did a few surveys and got the game free, but overall this game is great.