Why does paul not want Miranda to say anything about the dead rabbit?

Miranda and Paul live on a farm, so hunting animals is pretty much a way of life for them (although Miranda only accompanies Paul on his hunt so she can go for a nice walk in the woods). Killing animals is a normal part of living in the country, and Miranda's none too squeamish about it. That said, she only really has a gut instinct for killing snakes, as they're considered dangerous.
During the hunting expedition/walk in the woods, Paul shoots a rabbit. When she gets up close to take a look at her brother's handiwork, Miranda's pleased by what she sees. She watches admiringly as Paul skins the rabbit, and she even allows herself to touch the dead animal's exposed sinews.
But then the atmosphere suddenly changes as the two children realize that the rabbit was pregnant. Miranda's still fascinated; she gently touches the dead babies that look so pretty and peaceful. Paul, however, instructs Miranda not to tell anyone about this. He puts the babies back inside their dead mother's womb and hides them all behind a bush.
Paul's trying to protect his little sister from the harsh realities of life. In effect, he wants her to bury the experience of seeing the dead baby rabbits. He is disturbed by what he's seen and doesn't want Miranda to feel the same way. Ironically, Paul, in not giving the dead rabbit and her babies a proper burial, has treated them in the exact same way that his grandmother treated her late husband's body. But the memory of what Paul and Miranda saw cannot receive its own proper burial, at least not yet—not until they have finally matured and come to a deeper understanding of death's true significance.