Fender - your OP is somewhat misleading, because it infers that by simply writing a letter, this, or any other child, somehow converts to Islam, as if by some magic spell.
Encouraging young people to explore different ways of living from their own assists understanding of other cultures.
Given that it is the dreadful levels of ignorance that drive extremism in the first place, the notion of examining something and thinking about it cannot be a bad thing.
I think this parent's fear is unfounded, and his reaction is ill-judged, but that does not mean he deserves abuse from strangers because they have similarly misunderstood his motives in exactly the same way as he has misunderstood the school's approach to teaching.
Modern children are perfectly capable of assessing something from an intellectual perspective - there is no indication that writing a letter from a different viewpoint will automatically lead to a wholesale conversion to another faith.
If a child of mine brought home this assignment, I would be keen to discuss it with them from a viewpoint of curiosity for people who are different from us, not leaping to media-fed paranoia and bigotry.
That's what extremists do - who wants to have a child growing up as ignorant and fearful as them?