Thank you for your query.
I can understand your concern but I will not be able to provide a definite response to your query without having a look at the red spots. They can be of viral origin and your son may be having diarrhoea
of viral origin whereas your daughter has escaped it. I suggest that you wait for another day or two and in case they increase in size then you should consult your doctor.
I hope I have answered your query to your satisfaction.
Wishing both your children an early recovery.