It is a wet winter Saturday on a muddy playing field in an unprepossessing part of south Manchester. The river Mersey slides past behind a row of trees planted to provide shelter from the wind, chill gusts cutting through the gaps where saplings were torn out by local kids to sell on elsewhere.

On the pitches, children of every size are playing football: four- and five-year-olds being taught the game, three teams of under-nines, the same again of under-10s. Dogs are being walked, and dogs are being allowed to do what dogs naturally do. By the changing rooms, a woman collects subs of £2 a child, less if they have brothers or sisters playing and the extra cost would mean one of them missing out.

Old Trafford lies four miles and several worlds away, across the flat suburban streets of Stretford to the north-west. The Etihad Stadium is six miles to the north-east, beyond Rusholme and Ardwick, new oil-money-bright in an old coal town. And yet these council fields are the new front line in the battle for supremacy between United and City, and these kids – shivering, laughing, falling over and pushing past – are the trophies both clubs are fighting for.

The reasons are not hard to find. Inside the squat changing room, away from the damp patch on the ceiling where the flat roof leaks, a trophy cabinet spills its silverware on to shelves and filing cabinets either side. On the opposite wall are photos of the young-boys-made-good who won them.

Marcus Rashford, striker for United and England. Danny Welbeck, United, Arsenal and England. Wes Brown, Jesse Lingard. Ravel Morrison - made good, made bad, as innocent here aged eight as he would ever be.