Prince William and Prince Harry certainly did not approve their father's infidelity, that even on the day Prince Charles got engaged with Camilla Parker Bowles, they had nothing but the same hurtful emotion.
The story of Princess Diana and Prince Charles is comparable to those stories written on the children's books which started with "once upon a time." The only difference is that it never ended with "and they lived happily ever after."
Prince Charles did not want to marry Princess Diana because he was in love with Camilla. His dad, too did not want to marry Queen Elizabeth II, because he was in love with someone else.
The royal family was unwelcoming of the arrival of Camilla Parker Bowles to the Royal Palace, and it didn't help that Prince Charles delayed his engagement to his now longtime partner.