I've also now written a driver for the TY-HG06338 Smart USB Extension, which took a little longer as I've never written a driver with child devices before. Everything now works with no spurious children created during setup.
The weird 'F2' endpoint which gives rise to a useless child device on generic drivers looks to be a firmware bug. I think it's supposed to be reporting 'FF', which is the global on/off control which mirrors pushing the local button. However, the local button reports as endpoint '01' which is not very helpful as that's the same as the first socket. It's all sorted in the driver though.
Essentially the driver gives individual control of the three sockets as child devices, global on/off on the parent and presence detection.
