To be honest if I was walking with a small child in a buggy or walking alongside me, I'd be hesitant about going out into the middle of the road to help someone. I'd certainly do *something* though.
A few months ago I saw someone taken off his bike by a car that turned right straight across him as I was walking my son to nursery. I went to make sure he was okay, but by the time I'd safely gotten my son across the road, he'd already got up with his bike and the driver of the car was out helping him as well. (Incidentally, the cyclist escaped with nothing more than a skinned elbow, and seemed very calm, almost apologetic about the whole thing)
My guess is that all those passing mums assumed that someone not currently encumbered with a child would help. I seem to remember hearing about some research showing that if someone is hurt they'll get assistance more quickly if there's less people about because people are less likely to assume that someone else will do something.