Gryphon,
This behaviour was observed by (if memory doens't fail me) the Joubert couple who live in the Lynianti/Savuti area of Botswana. Belive it or not it was part of a National Geographic documentary.

The behaviour was filmed in a water hole that was drying through the long drought that lasted a couple of years in that area. And as water was finishing the adults did not allow the small ones to drink.
DDouble