There is a Santa. He isn't a fat man in a red suit, but He can travel all over the world instantly and brought us the greatest gift ever. And that's what I tell my kids.
My Mom was raised Catholic, in Austria, and "Santa" was KristKindle - The Christ Child. We wrote letters to Santa and put them in our windows for the angels to collect, and if we were bad, there was always some angel looking in the window just then...so we tried to be REALLY good.
My Dad and Grandfather would "go out to check the cattle" on Christmas Eve...just like Dad did every night, all winter. It took them seriously forever to get back, something about the animals being shy because they were talking, of course. Then Dad would burst in and say he was sure he'd seen Santa on our roof as he was walking into my Great-Grandparents' house, and we were allowed to go find all our presents under our tree.
It was great!! We still miss those Christmas Eves and wish we could recreate them for our kids...
I know my older kids decided there was no Santa at some point growing up...but weird things kept happening that made them at least think. A couple years, gifts showed up in our locked house while we were gone, and all the doors were still locked. Even when they knew I was flat broke, there was at least one family gift from Santa, something we really 'needed' like a VCR. One year, there was even a gift for me because I needed a winter coat. (

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But the really big year was the year my most-doubtful found a heavy brass sleighbell in our front yard. We still have that bell, and it's no cheap little thing. I bet a horse could step on it without breaking it...and it was in the yard right where it would have fallen if Santa had been on the front porch. There was no Christmas parade in our town, and no reason for anything like that to show up at our house. (I most certainly had nothing to do with it!!) After that, they all had to wonder...