It's not well promoted. It's not on ticket machines. I've travelled a fair bit in Greater Manchester (sometimes with a child) and never come across it.
That's poor. It's also not offered when you do a search for a fare for an adult and a child for any journey within the TfGM area on the Northern website, although I suppose that's understandable as AIUI it's a Transport for Greater Manchester product rather than a National Rail or Northern product.
Even so, not being able to get it from a ticket machine means that an adult accompanying a child boarding at a station with a machine but no ticket office (or with the ticket office closed) would end up paying more than they need to if they don't know about this offer, or, if they are in-the-know, they could be faced with a dilemma of whether to buy their tickets from the machine and pay more than necessary, or get on the train and risk being apprehended by the guard for not buying before boarding when they could have done so (or worse still, being stung by the revenue protection squad on the barrier at Piccadilly or Victoria if the guard doesn't come round on their train).
If you have a Family & Friends Railcard, you also need to check whether railcard discounted tickets will be better value for the journey you are making (they will be for some longer journeys within Greater Manchester). This offer is not available if the accompanying adult has a rail rover or ranger ticket, a DaySaver or a free travel pass, but I presume it is valid if the adult has a railcard other than a F&F in which case the child would not ordinarily receive a discount on the normal child fare (such as a 16-25, 26-30, HM Forces, Senior or Disabled Railcard, or a Two Together Railcard if it's two adults)?