I went from a 5Dc (which had the same AF as the 5D2) to a 5D3 (which has similar AF to the 5D4). I did some manual focusing with the 5Dc because of the limited AF points and the less sensitive low light capability. Again, I think the main deal with AF speed (in daylight) is more to do with how much focus pull it’s having to do (example being wildlife, where you might switch focus from something far, to close, to far again). Personally, I prefer the 5D3/5D4’s use of more AF points. I got a 7D2 for a crop since it has more AF points and an interface like the 5D3. I know some people have said they don’t use the 7D2 as much if going with a 5D4, as the extra resolution is more tolerant of cropping.
Extra AF points helps with normal focal lengths where I can select a particular point I want to focus on. It also helps with telephoto: in your example of a kid running up to the camera, a camera with 61 AF points has more spread for AF tracking and keeping points on the kid.