My contract did not include anything about photo usage so I email parents for permission as needed. Like sunnydays though, I never include faces in photos that will be shown publicly. If the faces are in the original photo, they will be cropped out. Once I have selected and edited photos for privacy, I attach them to an email to show the parents and ask if it's ok to use them.

I stress that the photos will "always be edited in such a way that faces are not showing.". I always ask in a way that they may say no if they don't feel comfortable with it. They have always granted permission readily though.