Princess Diana’s sons were kept away from her casket for the two days between her death and the beginning of the public observances. The decision was made by the palace. Neither William nor Harry was consulted. Diana died at the Pitié-Salpêtrière Hospital in Paris at 4:00 AM on August 31, 1997. Her body was repatriated to England that afternoon and taken initially to the Chapel Royal at St James’s Palace. She lay there privately in the days before the public funeral.

William and Harry were at Balmoral when the news reached them. Prince Charles flew immediately to Paris. He returned with Diana’s body. The boys remained in Scotland with the Queen and Prince Philip.
The decision to keep them at Balmoral — in the environment they knew, with family around them, away from London and the accumulating media presence — was made by the adults around them with what those adults believed were the boys’ best interests in mind. It reflected a particular philosophy of how children, and specifically royal children, should be protected from the direct experience of events beyond their capacity to process.

Harry has written about those days at Balmoral in Spare. He describes them as days of unreality — moving through rooms and routines while the world outside processed something he had not yet been given permission to fully feel. He describes wanting, and not being permitted, to see his mother.
He was twelve years old. His mother was in a chapel in London. The decision about whether he could go to her was made by people who were managing his grief and an institution simultaneously.
He did not go. He has written about it. The people who made the decision have not publicly addressed his account of it.