Lying about Santa can be a sensitive topic. For some people, this sensitivity has something to do with the figure of Santa himself: for them Santa is sacred, a bit like God, or the government: a basically magical entity that lives far away and gives them free stuff, and forms part of the basic fabric of culture and life without which the world would be a rather colder, nastier, more inexcusable kind of place. Others are sensitive about it because it concerns their own parenting decisions, and since parents naturally want to do well by their children, it can be uncomfortable to consider that certain treasured elements of one’s parenting practises might in reality be morally suspect.
In any case, sensitive issue or not, I think it's an interesting one, morally and sociologically. Morally speaking, we generally recognize that it's wrong to lie (and yes, wrong to lie even to your own children). And yet in our society we have a generally accepted practise of lying - entirely gratuitously, I might add - to our children about Santa. So why do people intentionally deceive their kids into believing stories about the jolly fat man with the beard and the reindeer? It seems obvious that the primary reason they do it is simply because "everybody's doing it" (it's a 'tradition') - and practically speaking, that is reason enough.
But if you ask parents the straightforward question, so why do you lie to your kids about Santa?, you might find - as I have - that they are surprised and offended that anyone would even say such a thing; and they might even deny that it is lying - even though it perfectly obviously is. (They knowingly assert falsehoods to their children with the intention of inducing them to believe that those falsehoods are in fact true - in other words, it's very straightforward: they lie!)
But I guess most people would not care to outright deny that they are lying to their children, so instead some justification needs to be offered for such an abuse - I'm just saying! - of their children's confidence and naïveté.
The primary reason why people actually do it (lie about Santa) in the first place is, as mentioned: everybody's doing it; and this reason might well be offered by some people also as a moral justification. But "everybody's doing it" is a pretty lame moral justification (obviously!), so in spite of the call of the herd, they need to appeal to more than just that.
So what more is there? There are a series of rather vague notions: it's harmless, it's fun, it's part of childhood, it's good for children to have vivid imaginations, to believe in magic, etc. And because everybody's doing it, they don't want their kids to miss out on the fun. In general, then, the idea is: I don't want to deprive my children of living this (lie-based) fantasy. So sure, I'll lie in order to give it to them.
Now I love fiction. I think it's mostly harmless, often beneficial, fun, part of childhood and of having a vivid imagination, and yes, everybody exposes their kids to fiction, and I wouldn't want mine to miss out on that. But still: If my son asks me, say, is Narnia real?, I answer him truthfully. When a child asks for the truth, I'm pretty sure that is a sign that he wants to know the truth and is ready to be told the truth (in a way, obviously, that is appropriate to his level of maturity and understanding, and doesn't simply denigrate the value of imagination). So I don't worry about whether I'll be stunting his imagination by revealing the truth to him when he asks about it, and I certainly don't think that any such concerns could justify outright lying to him about it. And I don't think there are any good reasons for thinking that the same doesn't apply when it comes to truthfully answering questions about Santa (not to mention introducing your kids to Santa in a truthful way to begin with). But it should go without saying: if there are any such reasons, please, I'd like to hear them!
Nonetheless, there is one obvious 'justification' for lying: I need no justification, because lying isn't wrong. If you don't like lying, then don't do it! But mind your own business: you have no right to arrogantly impose your moral views on others. Given the evident brazen hypocrisy of saying stuff like that, you would hope that no one would ever actually take such a position. But in the real world, unfortunately, shit (i.e., post-modernism, psychological trauma, nihilism, narcissistic personality disorder, bad parents and teachers, etc.) happens, and people all too commonly do have this kind of angrily puerile reaction when their views are challenged. In other words, the issues in this case are likely deep-seated psychological ones, not simply matters of confused thinking - and sadly I'm not much of a psychotherapist.
However, for those not overly hampered by arrested psychological development (such as - dare I say? - myself), it's often helpful to think about what a bright guy like Thomas Aquinas had to say about the subject. So put this (Summa theologiae II-II, q.110) in your pipe and smoke it (in honor of jolly old Saint Nic). Some quick highlights