There's no perfect solution, because the jaw action is a morph, not a joint.  

The easiest imperfect solution is as follows:

1. Parent the teeth to the head.

2. Show the green cross-thingie for the Origin of the teeth.  
   (Either hit Alt-W.J on the menu, or check the "Display Origin"
   on the parameter box)

3. Pull the green cross back to the approximate location of
   the jaw joint.  (More or less shown in the picture here!)

4. The hard part: Use the LipsOpen morph to open the jaw
   to various angles, and fiddle with the Yoffset and Zoffset
   until the teeth seem to follow the lower jaw closely enough.

5. Resave the figure with the prop attached.

6. Optional: If you want to make the action automatic, you
   could try using ERC to move the teeth along with the
   OpenLips morph.

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