In the past I had long term loan of a tandem with kiddie ranks. This was an additional set of cranks at a high level and a chain connecting the additional cranks to the normal rear position. There were also side bars to prevent the child falling out.
Fitting Kiddie cranks will not solve this conundrum.
But thinking laterally, as a low down ,layabout cyclist of the recumbent variety, maybe a recumbent of some design would suit.
Speak to Kevin at D.tek recumbents, he is a cornucopia of odd design bikes,
Best to telephone, he is notorious to contact, but ever so helpful and friendly to deal with.
In the end ,it's going to be a special needs machine that's going to help here.