Awesome! I really wish I had started at an early age. I didn't even know what triathlon was until I was about 28.
There's a 10 year old girl in Colorado who is often at the big races beating all the older folks. They keep her gear fairly simple, but they also do a lot of relays with her. Her dad likes to let her do the bike and swim, and in smaller races, she does the bike. The bike course can get intimidating and fast (same goes for the swim I suppose), and there are also a lot of races that do not block streets off entirely for races. Therefore it can get pretty dicey for youngsters unless the bike is on a closed course.
Another cool option for young people is indoor triathlons. They do a pool swim, stationary bike, and treadmill in order to start getting used to those requirements.
A good starting point would be a simple, small, local sprint triathlon. Try to get your child on a relay team to share efforts, and eventually transition (no pun intended) into doing all 3 legs of the triathlon. Finally, you can check out my site in my signature, it's got resources and a book for people brand new to triathlon. It is a super short read and includes info on how triathlons work, transition, stretching, and what triathlon gear is needed and what is not. Note that the training plans are designed for adults, and kids should not train the same way that adults do.
Nourish - Sports Nutrition Made Easy