Our Dr in NZ prescribed Slipper Elm powder, you can get it from the health food shops and you give them 1/2 teaspoon twice a day mixed with their food. I have found it fantastic (I always take some when we are travelling now), its great for when they are teething too. Dehydration is the big thing to watch for, try and make sure they are getting lots of fluids.

Good luck, its never fun when they are sick!