Why would you tell your kids there is no Santa?
Because I don't like lying to them?
I've let them all come ask me/us on their own accord and then just level with 'em. My twins are 8 and the last to "believe" but I think deep down they known the truth and are choosing to believe, which is fine. I figure this is the last year they keep the faith but it's up to them.
I don't advocate lying - and the situation you described is similar to our own.
But, I assumed this was more about just flat out telling them - as opposed to responding honestly to a question.
Though, if they were really young <4 - I'd probably answer a question with a question, and let them draw their own conclusions.