He tried killing his niece in 4, who was pretty young.
True, though family is different for him, and he waiting until Lauries son was 16 before trying to kill him.
I don't have the exact quote with me, but Stef (the writer) pretty much explained that this occurs during the time that Michael thinks Laurie is dead, so he's essentially entertaining himself during her absence.
Why? Last time something like that happened he went into fugue state for 15 years, which sort of made sense.
He's not so much interested in hurting the kids (which is why he doesn't kill them when he easily could)
That almost makes it worse, killing is what he does so there's plenty of precedence, and he's usually rather straightforward about it. Hiding razor blades in sweets just to hurt random kids seems too complex and pointless for him.
but is just messing with Mrs. Mackenzie. His version of Michael (which is a bit different admittedly than the films at times) is pretty sadistic and mean-spirited and enjoys scaring people just as much as he does killing them.
Then that just makes it... not Michael Myers to me, so why bother making it him?