If you want to open the Magic Kingdom, you have to be there very early and get chosen.
The other way is to ask to see if they will.
That is what we did.
My oldest wanted to open the park, so I asked a CM and we were told we could do it later in the week. They even changed our character breakfast.
We were lucky, the CM took one look at my boy and knew something.
I was so grateful, since the year before my youngest was chosen to be in the MGM parade and also the Lion King show.
My advice, ask when to get to the MK and be prepared to be flexible if they say yes for later in the week.
It was worth it.