What tool is needed to adjust this headset on a children's bike? My daughter's bike has a threaded headset, and I don't know what tool I need to adjust it. As you can see from the picture, there are the following parts in the stack:


a knurled ring, which presumably isn't very tool-friendly
a ring (with a diameter of 35 or 36 mm) with three notches spaced equally apart (I'm not sure whether or not this is permanently attached to the knurled ring)
a standard six-sided nut, which I assume I can loosen and tighten with an ordinary crescent wrench


It looks like I'll need some sort of special tool for the ring with the three notches. What tool will I need for adjusting this headset?
The bottom ring is used to provide tension against the bearings to stop the headset from wobbling around. It only needs to be hand tight, hence no spanner/tool attachments.

The middle and top ring are tightened against each other, as a lock ring to prevent the bottom ring from loosening.

The top ring can be tightened with any spanner of the right size.

The middle ring requires a specialised spanner called a C-Spanner, a hook wrench or a hook spanner. It looks like the image below and is readily available from any bike shop or online store. Eg: Chain reaction sells one from Park tools