I have a lot to say on this subject, but am away from a real computer today and a lengthy response on a treo isn't feasible. start w/ sheldon brown's website.

we have don 50 - 60 milers w/our daughter on a tandem, starting when she was about 7. (see my avatar picture....)

I will give more suggestions later, but start w/sheldon brown - he has a whole section on cycling w/kids on tandems.