I think you are assuming they would all want to be king / queen? I can imagine that over the generations there have been times of doubt for the intended monarch and a sense of jealousy towards the younger siblings for the relatable freedom they have without the scrutiny of being the future king. Look at Andrew, even before the recent issues he has had a luxurious life of parties and boats without ever being told ‘you’ll be king one day, you can’t be seen to XYZ.’
I always think the younger ones have it far better - all the money and the privilege, but without the responsibility. Then again, I wonder if part of Harry's going his own way is because he (and Meghan) doesn't want to just lie back and count the money without doing anything significant in his/their own right.
Nevertheless, being royal is the ultimate in 'golden child syndrome'. When Charles gave his speech and mentioned William as his heir, I was glad he mentioned Harry soon afterwards. Speaking as the King, William is his only heir; but as a father, he has two equal heirs.
I agree that they would have been told the situation from birth, though. The whole thing about royal life is that it is so very different from other people's lives. My DS can go to school, to a party, to the shops and absolutely nobody cares to the extent that they would want to take photos and videos and report it to the world. With the Cambridge kids, that's a certainty.
It's got to be just what you're used to, I reckon - and there isn't just one single chasm between royal and non-royal. I always used to be astonished (still am!) with Enid Blyton books (yes, I do know they're fiction) when the children always took it for granted that they had a driver, a cook, a maid, a governess etc. (as in specific employees and not their mum and dad) as those kinds of things were just so alien to me. Then again, if I wrote something and commented mundanely about a character popping to the kitchen to get a drink of tap water, there are countless millions of people around the world who would be shocked at my being so nonchalant about (for them) maybe half a day's journey apparently taking half a minute.