Final answer:
The best action is to discreetly allow the grieving parents to see their deceased son, maintaining privacy and dignity for the family.
Step-by-step explanation:
When the child's parents arrive at the scene and demand to see their deceased son, who has suffered numerous disfiguring injuries, the most appropriate course of action would be option D: discreetly escort them to the back of the ambulance and allow them to see their child by themselves while you wait outside. This option respects the parents' need for closure while also handling the situation with sensitivity. It's a very delicate and personal moment, and the presence of a professional who can support and understand the gravity of the situation can be invaluable. In the context of the given scenario, it is critical to tend to the emotional needs of the grieving parents with compassion and dignity, while also ensuring the respect for the privacy of their deceased child.