I know two people who have had this. Both, I'm sorry to tell you, were insufferable brats.
The first was a former housemate of mine from when I was 20-21.
His parents told him when he was 18 or so that they would buy him a flat outright, although they didn't specify when. While I was living with him he found a place he wanted, and approached his parents, who apparently felt it was a bit cheeky for him to say "I'd like my flat now please" so they said they'd make him wait a couple of years. I think they thought if he had to wait for it he'd feel less entitled. It did not work, unfortunately.
While I was living with him he worked as little as possible, making literally the bare minimum required to pay his rent. He did ok at school but had no plans to do anything else, and wasn't interested in uni. He thought of himself as some sort of artistic spirit, and told people he was a photographer. It is true that he owned a really nice camera (which his parents bought him) and wandered around London barefoot (seriously) taking pictures which I think he thought were very arty and interesting, but were actually a bit shit. He never made any money from photography. A couple of years later his parents bought him a flat outright in Islington. He immediately rented out the spare room and the living room, used that money as his income, and went on being an "artist". He then decided he wanted to be an actor. Went to a few acting classes and joined the worst improv group I have ever seen in my life. Did a couple of student films. Got bored with it. Now mostly spends his time updating his word salad "artist" website and chatting up the poor unsuspecting women of north London, telling them how photogenic they are and pretending he used to do Shakespeare. I've known him for 20 years now and he has never, to my knowledge, had a proper job.
The second was the heir to the family business. Before buying him a house (!) his parents paid for his rent, bought him a brand new car, and paid for his petrol. This was because he was living in London and they thought, well, London is so expensive, this will give you some freedom. The freedom it actually bought him was to sit around and play X-Box all day - and I really do mean ALL day. He would demand things from his parents and was unpleasantly mocking to his mother, who (having retired early from a career in the NHS) was always dropping everything to do whatever he wanted, ferry him around, buy him xyz. If on the rare occasion she said no, he would laugh in her face and say "Be serious, what else are you doing? You don't have a life!"
His parents created a "job" within the family business which required him to be in the office one day a week, to ease him into it. Since the role was created for him and wasn't really outlined (or necessary), he mostly scrolled Facebook and watched YouTube.
He got married after a whirlwind romance, and his parents bought them a three bedroom house with a garden, outright, in his hometown. Shortly after this, he took over the family business. Shortly after that, 90% of the staff - who had been there for decades - quit, because he was treating everyone so badly. It's the kind of business that will always be in demand, so it probably won't ever go under, which is almost a shame because frankly he deserves to fail.
The standout detail with the above examples is that both knew from an early age that their futures were sorted and they didn't have to work. Despite these cautionary tales, I do actually think this is a wonderful thing to be able to do for your children. But I don't think it's wise to do it straight away. I think a bit of struggle in your 20's really gives you some drive, a strong work ethic, some perspective of the world and how hard life can be, and a genuine appreciation of such a generous gift when it does come along.
If it were me, I'd wait until they were closer to 30 than to 20. Let them live in grimy flatshares and have to go round Tesco with a calculator for a bit. Don't tell them what you're planning for them. Honestly, it's character building.