How long does a seattube need to be to keep the kidcranks from clipping the regular cranks below?
If you can run them with equal sized chain rings there's no problem. The other trick is to obtain an old chainset and saw the cranks off. Otherwise it depends on the combined crank lengths.
You can use a longer seatpin. For a young child you can fit a second rear extension and bars to the rear seat pin and connect them to the stoker bars, using tubing of some sort (straight mtb bars?), to make a safety cage.
