My daughter is 12 now and has been riding with me since she was 6.
She regularly gets dropped off to school on the bike and has been on runs up to 400miles/day. She is currently clicking up miles at about 5000/year and has probably done about 20k miles total.
She has all the gear and knows how to use it. She has ridden in rain, snow, hail and 105'F days.
She has the bug and would not give riding now, she wants her first bike to be an ST! I think I will start her on something smaller.
The bottom line, treat their gear the way you treat your own. Once you know they are going to ride regularly they will need a full face helmet mesh jacket, winter jacket (if not the same), rain/winter pants, good boots and gloves. The only "problem" is they grow out of it quickly and it will need updating. Try and find buddies with kids that ride too.
Start them on short trips at first. Always build in extra stopping time on longer trips until you know how well they will cope with it. Like any road trip they will likely get bored, bring along an mp3 player, and look for stops of interest to them.
-- Mark