I see many threads each year where people are upset that a child has worked out that there is no Santa. But my 8 year old worked it out and I feel differently.
It's like a whole level of stress has been removed from Christmas. No hiding things at relatives houses, we can just say 'stay out of that cupboard, the Christmas presents are in there'. No trying to explain why he got four times as much as one friend got, but only a quarter of what another friend got. No having to hide different rolls of wrapping paper so that Santa doesn't accidentally use the same paper as mum and dad.
I know a lot of people feel sad at the thought of their children growing out of things, but honestly for every negative there is a corresponding positive.