What Causes High Platelets in Children?

My nine-month-old son had a fever and viral cold with throat ulcers and was hospitalized for six days. When discharged, his blood platelet count was 500, and it was 545 the next day. The doctor suspected ‘Kawasaki disease,’ but there were no symptoms of Kawasaki disease. I’m desperate and don’t know what the cause is. I hope for an answer, thank you!
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