I do not think that is universally true. My experience is that opioids make me feel weird and have all kinds of side effects. I would be hard pressed to say they actually have any effect on pain. I have told doctors outright that I don't want a narcotic. I do not think that I am the only person for whom opioids have no pain relieving effects.
With a non-verbal child, how can one determine if the pain is actually alleviated, in a way that distinguishes pain relief from drugging the child to the point of being unable to respond yet still in pain?
I don’t know the science of pain meds. But an oncologist I once knew said that, after 40 years’ experience with cancer patients, he had never seen intractable pain, but he had seen intractable doctors and nurses.
You’re always aiming for “calm and alert” and a pain level tolerable to the patient, and that takes very careful dosing. Patient self-dosing is often the best, because most people will choose “calm and alert” some of the time, when they want to interact with people, and well-sedated otherwise.
Even a level of (say) morphine that measurably depresses respiration and predictable shortens overall life expectancy, is ethically justifiable if it is necessary to stop intolerable pain.
About opioids and pain ... I wonder, too. Maybe it’s just me. I had morphine during labor for Baby #3. The effect was that it still hurt, but I didn’t care. For severe pain like bone cancer, you have to wonder how much pain the patient still has.