The knurled part is the top cone, the three-notched piece is a locknut. There generally should be a tabbed lockwasher between them that fits into a notch in the fork column threads. The angle of the pic does not allow me to see if there is a notch on that fork. If you tighten the knurled part down by hand and still have play then there's something wrong with how the bearings are set up - balls are too small (some of the kids bikes took 3/16" bearings instead of 5/32") or the bearing cages are reversed, or even one is missing.

Once you have confirmed that you can remove all play by tightening the top cone or corrected any problems that prevented doing so I would just dispense with the notched locknut. Substitute some plain washers to occupy the same thickness. Install the front wheel to make it easier to hold the fork in position (by placing your legs on each side of the wheel as you stand in front of the bike). Turn down the top cone by hand till it stops and then back off about 1/8 turn. Hold it in position with large vise grip or channelock pliers (pipe wrench is possible but would need to be attached in the counterclockwise position) and tighten the top locknut. Check to see if the fork turns smoothly with no fore-aft play. If not hold the cone in place while you loosen the locknut, readjust the cone and retighten.

It's possible to adjust using the notched locknut, but it will require being able to both hold the cone in position and firmly tighten the locknut. The hook spanner normally required to do that is a rare item, even in a longstanding, well equipped shop, and using a visegrip will be difficult when you are trying to hold onto the cone adjacent to it.