Can you try moving the automations using original child device (one with User type) to use new child device and then deleting it? I'm sure that neither system parent device driver nor child user driver are written to handle this situation...
Looks like the driver you are using now has children.. Try switching to "Device" driver, it will let you remove children.. Then you can change to the new driver and hit Configure() and it will create the appropriate children for you.
I was able to remove the rogue child device using the driver called "Device". When I try the Hubitat driver shipping in 2.2.9, I cannot control the switch from within Hubitat from either the child switch device or the parent device. If I switch back to the community driver it all works fine.