Yes, it is moral and rational.
There is nothing immoral about the Santa thing. Lots of cultures have stories like this for children. These stories serve a cultural purpose in raising Children. That's how I looked at it, anyway.
I told my kids that Santa was real. I told them how much Santa loved the children and wanted them to be good, blah, blah, blah.
Then, when they got older, they confronted me about it...and I told them:
"Yes, Santa is real, but now, by asking these questions, you show that you are old enough to know who he is. You are old enough to know the secret. You already know Santa keeps track of how you are doing, he loves you, he provides you with wonderful things. Now you are older...and you want to know if he is real? He is REAL. But the secret is that 'Santa' is not just one man...'Santa' is all the adults in your life who have been raising you and loving you! Santa is uncle Jim. Santa is your dad. Santa is me. Santa is the neighbor lady, Mrs. Jones. We are the ones who have been loving you and keeping track of how you have been doing and we are the ones who put the gifts under the tree. WE are Santa. And now, because you have shown you are old enough, YOU ARE SANTA, TOO! Now YOU are helping to raise the little ones, keep an eye on how they are doing as they grow, and you will secretly get them gifts at Christmas! Because now you are Santa, too! It is a serious responsibility, all this helping to raise little ones to be good, setting an example, and carrying on the Christmas traditions, but it is also so fun! Congratulations!"
It was awesome. And it was the truth.