You shouldn't make fun of people with a stutter, even if they are an asshole.
Let's define a Vulcan child.
The emotional development of an emotionally stagnant/devoid creature?
Logic trumps childhood.
The intellectual development of a creature with the potential for an IQ 4 to 5 times that of a "normal" human being.
So what we're really looking at here is Stewie Griffen.
A child's body with a completely adapted healthy Vulcan adults mental/emotional acuity and composure by the age of... 6 months?
A few years. Maybe three? I have met some really on to it human 3 year olds.
Here's a point that Peter David raised.
If the upper limit of their life expectancy is 300 years, and middle age is there for 100 to 200, then adult prime is 50 - 100, and adolescence 25 to 50, which means that childhood should be the first 25 years of their life before they begin to look like a human 10 year old.
Which is logical, but doesn't track with the available evidence on hand.