The leading cause of elevated platelets (this is called thrombocytosis) in children is infection. I think the presence of a temperature and rash makes this even more likely. Infections can include pneumonia, meningitis, among others.
Anemia is another possibility, as is leukemia or other malignancy, but these are far less likely than infection. There are other more obscure causes as well.
It sounds like the pediatricians and physicians attending to your grandchild are beginning the testing and treatment process. Hopefully, they will discover what the cause is in a short period of time and begin treatment as soon as possible.
I hope this helps. Please let me know if you have any further questions - I'm more than happy to help. Take care.
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