I did the exact same thing as MJ when I found out the truth about Santa from my dad. If it was the truth, then I thought it was the right thing to do to tell my brothers and all the other kids the truth. My aunt did the damage control. Here is what I told my daughter about Santa:
There really is a Santa Claus. Santa Claus is a symbol of generosity and love. When kids aren't mature enough to understand abstract concepts like this, then we have to make it concrete or give them a picture so they can understand it. Only when children are old enough are they told that Santa really isn't a man in a red suit, but a special way that people can be loving and generous without getting credit for it. But, it is a very special secret and only parents can decide when their child is ready to understand that Santa isn't a real man in a red suit, so it isn't fair for anyone but a parent to tell this special secret.
Then I reiterated to her the importance of keeping the secret and letting the parents decide when their own children are old enough to understand abstract concepts like Santa Claus just before we went to my brother's house to celebrate Christmas morning with his two young children. I also asked him and his wife if their kids still believed, just in case she spilled the beans, but she did not as far as I know.