I personally love them and have been to two or three of their concerts. Whether or not your young child would appreciate it depends on your kid. If your kid can't sit still for a very long time, then maybe take a friend instead of the child. The year we got floor seats, my SD was entranced because there was snow falling on us and she loved the lasers, but she was also about 14. They always sell out when the come to Baltimore.
I think their CDs give a good representation of the music. For most rock concerts, we do bring earplugs but it doesn't mean you don't get to enjoy the show. They ROCK and they are LOUD. I bet Metallica and similar are, too. If that's not your speed of Christmas music, skip it.
Now I want to go look up their tour schedule. We had a big family vacation so I don't think we can swing tickets (and a sitter...my DD is 3 and wouldn't last) but I'm a bit jealous that you got free tickets.