Don't worry, you just need to hold the button longer. First, make sure the purifier is running (it can't be in Sleep mode). Locate the button that controls Sleep and Child Lock.
Press and hold that same Child Lock button for 5 seconds. You should see the indicator turn off, and all the other function buttons will return to normal.
To enable Child Lock again in the future, press and hold the Sleep/Child Lock button for 3 seconds while the unit is on. This prevents accidental button presses.