There's a very little one near me who stokes (although that might be a bit of an exaggeration) with kiddie-cranks, but s/he (can't tell) has some sort of back-support so s/he's not sitting unsupported on the saddle. I think the child's two or three - old enough to talk. I suppose it might be four, but then it'd be going into nursery with its older brother.
Every time I see them (outside the Steiner nursery) I have an urge to rush over and inspect the tandem, but I think my intentions might be mistaken for an urge to rush over and inspect the child, so I have merely looked from afar.