Water, water and more water! Give them a REALLY GOOD drink, think like 2 litres each, given slowly over two hours, say 500ml evert half an hour. This will give the water some time to be absorbed. If you give them 2 litres in one go, most likely the water will just run straight through. After this they should recover by morning.
If not, send me a message and I will get back to you ASAP. Hope this helps... Organoman.