A guy I know has a tandem. He rides in the front, his 10 year-old daughter rides in the back... PLUS he has a trail-a-bike with his 5 year-old second daughter on it! And he does serious climbing with this setup, and with the two of them more than a thousand km per year. So it is totally do-able. Not sure how you get the tandem to fit two people of very different sizes, but I guess that's what they are designed for...