I thought the moment had come a couple of months ago. We were in the car and the youngest asked "Mum, do you think Santa is real?" i thought I'd test the waters so asked back "well, if he isn't, who leaves all the presents?" Eldest fired in "Maybe it's mum's and dad's" I was about to respond when youngest declared "Honestly C.... do you really think that Mum and Dad have that kind of money after paying for our holiday in Turkey?!!!!!!"
....and the moment had passed!
Eldest obviously has his doubts, but as you say it will probably string out for a wee while - I do enjoy it in a way, but I don't think they'd be any less excited and it would save a lot of hassle if I didn't have to hide all the wrapping paper etc.