Shooting kids with a fast 5cm lens wide-open is a tall order. Think like a street photographer. Shoot a wide lens (I use a 25mm Snapshot Skopar), stopped down to f/8 or f/11, focused at 4-5 feet, and shoot away. No focus needed.
That said, the Nikkor 50/2 is lovely, and at f/2.8 gives great results. Try setting focus at the minimum distance and moving yourself in relation to your subject to get focus right. It's easier to move the camera than it is to fiddle with the lens. Here's a link to a shot of my niece from yesterday through the Nikkor at f/2.8 using this technique:
https://www.instagram.com/p/CO4S5i_rb6_/?igshid=mnsax4ocw33j
Sorry to post an IG link -- I've been away from the forum for many years and forgot how to post images properly here.
Sanders
www.instagram.com/sandersnyc