Why all of this working? How can the work be explained, made interesting and relevant. Is there some way of involving the child that is educational rather than abusive? (Also, if a 5 year old can understand how to correctly use the UI for the application, why can't your client?)
Could you involve the child? Maybe, but training someone for work is non-trivial. When I'm just starting to train someone, our collective output is usually less than mine working alone, and that's for someone with the requisite skills. How much work could reasonably be assigned to children in a way which isn't just a waste of time? How much are they really going to benefit from that?
Still, I started programming and using UNIX when I was 9. I imagine I was probably capable of some amount of useful output by 11-12, though it would have to be carefully directed by a pragmatic mentor with a view to bottom-line impact.
From my side, in the company where I work there are a small handful of people who could easily be replaced with fully unskilled labour (ie. an 8-12yo) with no negative effects.
I wouldn't want to actually implement it.