The problem with the line of thinking that your children won't trust you if you tell them Santa is real, is that you are then kind of denigrating ALL the hundreds of thousands of parents who do. As if telling a child that there is a Santa would cause them to distrust you. My mother is as trustworthy and religious as they come, and I have never not trusted her because she told me there was a Santa Claus. I say this with the best of intentions, S., because I really believe that all the folks who tell their kids that there is a Santa, are okay in doing it. I'm not trying to ruffle feathers or hurt feelings.
Also, when kids get older, they figure things out in their mind, just like we did growing up. After watching Disney movies, Cinderella, Beauty and the Beast, etc., kids really do figure out later the difference in wonderful fantasy and reality. I especially like the idea of showing the story of the "real" St Nicholas that is the basis for Santa. But that's for when they are older.
I never actually had to tell my kids there was no Santa. They read the book "The Polar Express" and I put a bell on the frig. When they actually asked me if Santa is real, I pointed to the bell and asked them, "Can you hear the bell?". They never asked again. My MIL told me a long time ago that when her kids asked, she reminded them that there were gifts under the tree from Mom and Dad, and from SC. She told them if there were no Santa Claus, then there will be no gifts from SC. They didn't ask a second time, LOL!
Hope this is helpful to you as your children get older.