Int. Child Health Foundation promotes the use of Grandma's ORT (oral
rehydration therapy).

1/2 c. to 1 cup precooked baby rice cereal
2 cups water
1/4 tsp. salt (use a level measuring spoon or a "three finger" pinch)

Mix the rice cereal, water and salt together until the mixture thickens but
is not too thick to drink.  Be sure the ingredients are well mixed.

Give the mixture by spoon often and offer the child as much as he or she
will accept (every minute if the child will take it).  Continue giving the
mixture with the goal of replacing the fluid lost: one cup lost, give a cup.
Even if the child is vomiting, the mixture can be offered in small amounts
(1/2-1 teaspoon) every few minutes or so.  Giving Grandma's ORT may shorten
the episode.  Banana or other non-sweetened mashed fruit can help provide
potassium.  We strongly encourage parents to continue feeding children when
they are sick and to continue breastfeeding if the child is being breastfed.

This group has done research on what "clear" diets do a child's nutritional
status and how poor nutrition prolongs the diarrheal episode.

I think UNICEF or WHO has the info on the salt/sugar/water drink.  I think
there are even measuring spoons available so the parents measure accurately.