I haven't come across this type on a kids bike with three little indents in, what tool do I need?

Please note there is a top lock nut but I have lifted it out the way to show the threaded top race clearly, which is the unknown part to me!

I have tried tightening as much as possible by hand and then tightening down the lock nut with a headset wrench but there is still a fair amount of play in the headset so I'm obviously going to need a specific tool.