and a rash looks more likely due to a virus.
As you already know, most of the petechiae
do not blanch on pressure.
You may give her Benadryl
in appropriate dosage according to her weight. If after 2 to 3 days, if she continues to have the rash, better see your Pediatrician
How ever if she develops high grade fever/ lethargy/ oral intake is poor/ develops any bleeding tendencies do not hesitate to see your Doctor immediately.
Hope my answer helps you.
Wish a speedy recovery for your kid.