"Once a King or Queen in Narnia, always a King or Queen."
Another consideration is that, while Peter, Susan, Edmund and Lucy were given a coronation, they were actual Narnian rulers. Jill and Eustace were not. I think Lewis failed to give them a coronation on purpose. Do we Earthly "Kings and Queens" of God's Kingdom receive crowns before we reach Heaven? Not unless we're also Kings and Queens (specifically rulers) here.
Just my 2 cents.
