This shouldn't be done, it can cause diaoreahh (sp?) and tummy upsets. I'd go with the Er for IV fluids, without delay (dehydration is serious with anyone but especially babies).
When DS had gastro and was admitted to hospital for dehydration (he was barely weeing, wouldn't keep anything down and by the time he did wee it was the orangy crystal stuff in his nappy) he was nil by mouth while he received fluids overnight and then on half size bottles (usually 200 ml 4 scoops went to 100ml and 1 scoop) and this was upped gradually to make sure he could keep it down.
This was under the recommendation of 2 GP's and paed's at Westmead Children's Hospital.