It is powdery mildew. Whatever product you choose to use (if any) you will have to read the label to see if/when the pumpkins would be safe for consumption after application. Powdery mildew on its own does not make the pumpkins inedible. Here is a link for Powdery Mildew IPM though I will say you are at the point where sprays will do little to control the powdery mildew. In the future if it is a concern you need to plan for it early and be prepared.

great link weirdtrev.