I have participated in 3 tandem rallies now, and in each there were several families with kids on tandems in many combinations.

The kids look happy and the parents look happy. It seems like a great way to go. I've seen kids as small as 5 years old out there pedalling for all they are worth up hills. I think it's a great way to go and regret not having these options when my own kids were small.

go for it!