I think it would be very difficult to keep it a secret, this happened to our old next door neighbours.
It was 1997 and a couple of weeks before my 21st birthday, they won just under 2 million. Didn't go public.
Someone else did it for them and they ended up on the front page of the local paper, funniest thing was they got the woman's age very wrong saying she was much older. My parents think this bothered her more
The family were well off already, had the biggest and nicest house in the street and although they established new and very successful businesses out of it, the money caused huge trouble within the family.
I've seen them now and again over the years and are still very nice people and have done very well for themselves.
I wouldn't trust my kids to keep quiet though plus I live in a Close where the neighbours know everything and it's irritating.
We got new flooring and carpets last year the same week we were going abroad and one of the neighbours said "ooooh have you come into money?"
Yes we did, dp's nan had passed away and left him a small amount of inheritance.