Slightly piggybacking this thread as I think it's a broadly similar hypothetical.
The TfGM 'accompanied child' 80p rail fare (discussed previously
on these forums and subsequently in the press) is still not available at TVMs, as far as I know.
If an adult and child pair start their single journey at a station with a TVM but no ticket office, what's the best way to buy their ticket? Can the adult buy their own single fare from the TVM and then board without a child ticket (so that they can buy on board)? Would a Promise To Pay be at all relevant here? It's another case of "only partial retailing facilities available" and I can't work out what the 'right' thing is for the passenger to do.