Config AppleWatch to not dial 911 during "Fall Detection"?
Source:

Fall detection on your Apple Watch Series 4 is off by default unless you are age 65 or older.  There's no way to get Fall Detection to do what you want, it's not configurable that way.  I think your only option is to turn it off. 

From Apple's Support document:


  If you've entered your age when you set up your Apple Watch or in the
  Health app and you're age 65 and over, this feature automatically
  turns on.


If, for any reason, you decide that you don't want Fall Detection on, you can disable it in the Watch app on your iPhone.


Launch the Watch app on your iPhone.
Tap the My Watch tab at the bottom left.
Tap Emergency SOS.
Scroll down and turn off Fall Detection.  The switch is to the left when it's off.