Hi, thanks for using health care magic
Children are at increased risk for dehydration
during and following a bout of gastroenteritis
You will have to watch for signs and symptoms of dehydration such as decreased urine output, dry skin
,dry mucus membranes (eyes and mouth) ,excessive drowsiness
In terms of the rash, children may experience rash along with viral illnesses. They normally resolve soon after the viral illness and only symptomatic treatment is necessary.
If you remain concerned then it may be best to have her assessed by her doctor.
I hope this helps and you both feel better soon