If Kant doesn't believe that kids are completely rational then would he say that it is okay to tell them a white lie?
Probably not, because the concept reinforces the “lying to the murderer at the door”. If lying to a murderer at a door to assume you are protecting the person who is to be murdered instead of following a moral obligation that sets in motion a foundation of what is right and what is wrong, universally, then lying to children is a perfect example. That if one lies at all, the basis of morality has an imperfect foundation and leaves every concept of law open to be interpreted as hypocritical.
And even though lying to children may be some type of social hierarchy about society and it’s hidden logic, and that most people seem to find lying the only entertainment they have (my jaded opinion), lying is the same as violence, where it is not supposed to be accepted, but is accepted and disguised by our environment and can be used against us all, persuasively.
The argument against this Kantian philosophy is really just an argument that logic can be based on illogical foundations, which leaves the question open and almost paradoxical. To replace nazis with a murderer only helps Kant’s argument, that the world is open to tyrannical rule when this universal law is left to be interpreted abstractly.
In a letter to a pupil Kant argues that it is always wrong to lie. However, this makes his Imperative unworkable and I doubt he really meant to make it a blanket-ban. His Imperative suggests that it is okay to lie where you would wish everyone to do the the same under the same circumstances. In this case we can tell children about Father Christmas without worrying about Kant.