I remember my mother being angry when I said that Santa Claus was not true.

this realization may be good for developing rationalists, because it provides children with the experience of realizing that they hold beliefs which are wrong and absurd, and that they must reject them.

If parents reacted not with anger or disappointment, but with congratulations ("Well done! You passed the test! You've leveled up!"), this might have positive effects. I'm still mildly opposed to the deception though. Analogously: it may, occasionally, be optimal to hurt someone for their own good, but one should be extremely careful about that kind of action and very suspicious of one's own motives; we should be similarly careful with sabotaging another's map.