A is parent of B, let's say in primary school age. A invents fake profiles of n realistic, inspiring characters and starts sending anonymous letters from each of these n characters to B, at different times, with different styles, such that each character behaves according to their own profile and each with their own story to tell; B starts a private correspondence with some of them without realising A is behind them.
A makes use of these correspondences for genuine educational purposes (i.e. A is not a psychopath). Let us assume no actual manipulation is involved: each of the n characters will actually behave independently and without any hidden goal apart that of being B's good friend and having some good time together.
Of course, these characters may decide to uncover their true identity, more or less gradually, more or less explicitly, entirely or just partially, perhaps maintaining the correspondence and continuing it as a "game".
A's behaviour presents obvious ethical dilemmas (starting with children's right to privacy, their right not to be deceived/misguided). Can it be ethically acceptable or is there a way to make it ethically acceptable? Is it even legal?
Is that worse than parents pretending to be Santa Claus or the Tooth Fairy?
EDIT Following answers by jker and Ines, I try to clarify my question
What is the purpose of these correspondences? Expose the child to great personalities, fictitious but realistic, inspiring, able to communicate with the child and teach them something in a context that is outside what the child perceives to be "family" or "school".
Why, can't A do the same without this bewildering trick? Yes, of course, A can play one character: the parent. Unfortunately parents are not enough: all children need many good quality one-to-one interactions with as many different educators as possible, and from as many different cultural backgrounds as possible; however educators, relatives and friends are usually not able or not willing to engage, cooperatively with the families, in activities so much tailored on the needs of one single child. This trivial trick would allow the child to interact with astrophysicists, musicians, artists, philosophers, mathematicians, neuroscientists, etc. - and these would not be just "mum" or "dad", they would have an aura of magic, they would be fascinating - which parents cannot usually be. And having real pen-friends is such a great educative experience for children of that age, expanding so much their normal, little microcosms.
It seems necessary to highlight that the purpose is cultural, not psychotherapeutic.
Genuine educational purposes are perfectly compatible with having a good time as friends - I feel a bit uncomfortable having to state that.
Why secrecy? Because children need lots of good educators, and school educators, relatives and friends are not good enough, they are not astrophysicists, musicians, artists, philosophers, mathematicians, neuroscientists, etc. Often they don't even have time for individual children. For parents it can be much easier, they just need to get to know their child, pick some good book from the local library and write few pages per week in interaction with the child, mostly following the child's interests. If there is no secrecy or at least some degree of complicity from the child, the game cannot work.
"Genuine educational purposes" might count as a "hidden goal" (?), what is important is that the parent behaves ethically and does not attempt to manipulate the child in any unethical manner. An example of the latter would be a parent trying to extort information from the child or exert psychological pressure on them: that would be very, very, very unethical.
How long would the "deception" last? It depends on the maturity of the child (weeks, I would say, perhaps few months, just the time to conquer the child, convince the child that the game is fun). The deception can be removed very easily and the game can continue with more awareness (and more fantasy/imagination on the side of the child who would try to attempt impersonating their own fictitious characters, great exercise).
By the way, yes, I confess I do believe that children have the right not to be deceived/misguided, at least to some extent that is not easy to define/quantify.
Also, Santa Claus is a false persona, and I can't see any educational value in it, it seems to me a triumph of conformism. Children can be distracted from falling teeth in many ways other than the Tooth Fairy tale.