If police sent a 17-year-old to the hospital by ambulance how do you dispute the ambulance charge and whose credit will be damaged if it is not paid?

You don't dispute the charge. You thank the officers for looking out for your child, regardless of whether or not you feel it was justified.

It is unlikely there are grounds for dispute. The party who will be responsible for paying charges will be the parent(s) or legal guardian of the minor child. What actions might be taken if the charges are not paid would be the decision of whomever operates the ambulance service.