It depends on what you and your family and cousin believe in.
I've never really seen why swearing is offensive if you are just using it as a way to make your sentence more dramatic, since it is only just a few different sounds.
I don't think so. My cousin watches bleach with his dad, and he curses like a sailor. One Piece and Bleach are nothing alike, though. When I was 5, I watched One Piece. Now that I'm 15, I don't think that Bleach is appropriate for a kid. My dad watches it, and I watched a couple with him. It's too mature in many aspects for a ten year old.