Light Yagami/Harry Potter
It didn't take long for Light to hit upon the answer. Light had taught Dudley to speak clearly from a young age. Dudley did so in front of Light and also in front of his proud parents. Said proud parents had taken that as the sign of their darling Diddykins being more mature and intelligent than children his age and enrolled him a pre-KG program for gifted children. A recipe for disaster, if Light ever saw one.
Dudley Dursley was not well-spoken because of his intellect. He just tried very, very hard because otherwise he'd be ignored by his cousin.
Light sighed. There was no easy way out of this. He could help Dudley learn but one, he wasn't interested and two, that would only delay the inevitable. Dudley would eventually get dropped from the program and be advised to go to a school for normal children. Dudley would be happier but Vernon would be disappointed and Petunia would only cry foul. Not a healthy environment and mindset for a child to grow up with.
It seemed that in this household only he, Light Yagami, resident ex-mass murderer and sociopath, was the one who could contribute to the proper development of a four year old kid. What exactly had his life come to?