Yeah, unfortunately, kids are not always the perfectly rational creatures that we'd like, and this is where the nurturing parenting model fails and the authoritative one wins, hands down.
I mean, this is a
health decision.
Do you also ask your child whether or not s/he would "like" to have dental work done?
Wear protective gear when cycling or playing a sport?
Why not?
Honestly, I know that some kids are TRULY phobic about these things, and I'll say this-- I know JUST how "un-fun" it is to need several orderlies to HOLD DOWN a thirty pound child for a blood draw...
but the bottom line is that a CHILD isn't really capable of informed consent, or I wouldn't need to be her healthcare proxy. And if she isn't capable of informed CONSENT, then she similarly isn't really capable of informed refusal, either.
That's what she has us for.
Now, I realize that sounds harsh. Understand that there IS a loving way of firmly addressing this in a way that still respects your child as a human being.
<on child's level>
"{Name}, I am very sorry that you are afraid, and I understand very well that this isn't enjoyable. It would be nice to not every have to do things which are scary, uncomfortable, or even painful, but the adults who care for you and love you know that this is necessary. This is my decision, and while you may not LIKE that decision, it is final. It will happen whether you want it to or not. That is not what you control. What you can control is your own behavior. IF you can accept that and cooperate with the grown-ups during {procedure}, it will be less awful for everyone. I have {prize--I made it something that DD HERSELF had coveted} for you if you can control your behavior."
But you
have to follow through-- I had to NOT give a prize to DD after she kicked and fought like a thing possessed... just once, and then she understood that I meant it. This (seriously) worked wonders for my daughter.
After that, it was as if a light-switch had been turned off. She even placidly tolerated a truly heinous blood draw when she was six (multiple sticks in BOTH arms)...
She had her teeth gritted, to be sure... and kept steely eyes ON ME the entire time, talking to me about her PRIZE (a video game, at the time).
I'm so glad that we handled it that way-- because doing 6 years of immunotherapy would NEVER have happened otherwise, I can tell you. And that has made SUCH a huge difference in her quality of life.