Hi my family is suffering from something similar to the stomach flu diarrhea vomiting nauseousness but what is confusing me is that their platelets levels are dropping below the normal range, can you shed some light on this for me please?

I am assuming that if you have the platelet information then you're under the care of a doctor? There could be several reasons, but it would be reckless to assume one way or another. Please continue care with a doctor.

Platelet levels can only be determined by blood work up put through a lab. So how would you know this information for every member of your family?