All you need, is a large adjustable spanner that expands 30 plus mm, and large Channel Locks.
Usually I can take the loose headset adjustment cup(that knurled flange) and snug it up so you feel bearings get bumpy tight, just moderately tight or "binding" then snug up the top locknut with adjustable only maybe 1/4 to half turn after you feel it start to bind on the lower cup just mentioned.
Now you can take Channel Lock wrench and REVERSE or back turn it INTO the top locknut with you holding top nut fixed with the adjustable, it will tighten into locknut very well AND back off on bearing tightness so that fork turns fairly free, but with no bearing play at all.If end result still feels tight, then back off only SLIGHTLY on top nut so that you can also back the knurled ring adjustment bearing cup AGAIN, into the top nut, reducing the tightness affect yet still providing a free rotation of fork with no play.
If you take stem off, you can then get a sensitive feel for bearing smoothness. You really want it just slightly tight or "bumpy" so that fork can wear in, without working head bearings loose with the dreaded bearing play, that makes bike handle horrible and wear out bearings or even crack them. Loose headset also makes bike ride horrible too Too tight is no good as well, so keep a good feel for how tight or free the headset steerer tube bearings are.
Once locked down the bike will ride nice and keep lil rider happy and in best bike control. Good luck.
Last thought, I see the slotted lockring that can make things more complicated at first, if you have that maybe you better let a competent shop adjust bike headset and while at it make sure whole bike is mechanically sound and lube the chain.
Your lil rider with limited strength in legs will appreciated the solid bike with less drag.