4-year-old DS definitely does.
DD last year (then age 5) said to me "You're really Santa, aren't you." Denied it, then carried on as normal. Have done all the usual Santa and Christmassy things this year, she seemed to believe, then asked me again if I was Santa 🤷♀️
I think she's not sure, but definitely doubting. She's very astute though and sees things in black and white.... I think the story just has a few too many holes for her liking!
I think DS will believe for a fair few more years though 😁