Here is a link for oral re-hydration solution. It saves little ones lives in
developing countries and works here too (and is good for adults with
the runs)

http://rehydrate.org/solutions/homemade.htm

Even if they can't keep it down or in, the fluid will be helping purge
the gut and could shorten the duration of the illness.

The sugar helps the water to be assimilated more quickly.

If she won't drink, don't mess around and get her to the doc. It sometimes
takes only a day or two for those under 5 to succumb to that sort of
dehydration.