As you talk to your child, you can explain that in the recent years, you realized that we were all losing the meaning behind Christmas and discussed this with Santa (sometimes in life we all learn lessons, even Santa). You can support that your kids may have noticed that your family has not been spending as much, and there won't be as much spent on Christmas this year. They might notice that other kids may get more from Santa when they are getting less, but that doesn't mean that they haven't made good choices. The decision to spend less was made between you and Santa, and each parent makes their own agreement with Santa. If they feel upset that others get more from Santa, you understand that, but it is not a reflection on them.
The next issue is to help your kids to understand their expectations of what they will get by looking through a list of wants. If they are going to visit Santa, have them limit their lists to a few "realistic" items. I believe that having an endless list of wants that they can dream about only to find that they get nothing on that list leads to disappointment, sadness and bad memories.