This worked for me, unfortunately after I'd done a software reset which buggered up the home screen, Wi-Fi connections, accessories (including the Apple Pencil) and a host of other settings...
To get the Magic Keyboard Trackpad back to work - because the keyboard/keys were working, only the trackpad, I did the following:
Restart (hard reset using the power button), or go to Settings >> General (scroll all the way down) >> Shut Down.
Restart (hold power button for 5 seconds)
Go to General >> Keyboard >> Hardware Keyboard >> (Select your keyboard at the top) >> Automatic.
This should do it.
Then go back to General >> Keyboard >> Hardware Keyboard >> Automatic >> (Select your keyboard)
To get the Magic Keyboard Trackpad back to work - because the keyboard/keys were working, only the trackpad, I did the following:
Restart (hard reset using the power button), or go to Settings >> General (scroll all the way down) >> Shut Down.
Restart (hold power button for 5 seconds)
Go to General >> Keyboard >> Hardware Keyboard >> (Select your keyboard at the top) >> Automatic.
This should do it.
Then go back to General >> Keyboard >> Hardware Keyboard >> Automatic >> (Select your keyboard)