Being in Manhattan, probably was the sidewalk - this looks like a scattergun approach - sue every possible person involved - the shot will surely pick someone off.

I used to work for an insurance company dealing with liability cases - in this one the parent should have been exercising strict supervision over the child and not allow her to go racing off on her bike, both for the safety of the child and for other users of the sidewalk.

There is a duty of care to ensure that the actions of any individual do not cause a problem for any other user of a particular area. It's pretty clear that that did not happen here - any reasonable person should have been aware of the possibility of the child colliding with a pedestrian and kept the child in check.