Deaf children learn sigh language - which incudes the alphabet.
So tjhey know how to spell before hearing children (usually).
From learning the alphabet in sign language it is fairly straightforward to teach the written alphabet - it is simply a symbol that represents each letter eventually chained into words.
Of course,deaf children can face the same obstacles as hearing children - dyslexia and so on - but on the whole,deaf children learn to read and write quickly and easily, partly out of necessity.