I told my DS when he was 10. He asked so I told him. I was determined not to tell him around Christmas luckily he asked in winter. DD1 (10) 'knows' although I've never told her but DD2 is 7 and has no idea. It'll be a sad Christmas when none of my babies believe anymore

I'd wait a couple more years before telling her. I got away with saying 'if you don't believe he won't come' for a few Christmases with DS.