Dexter is the incredibly brilliant and incredibly arrogant American grade school genius with an inexplicable Indonesian accent. He somehow managed to manufacture a gigantic secret lab in his parents house without them even noticing. In this lab, Dexter works on countless experiments and tests. Giant robots, super serums, complex computer code... all these come second nature to Dexter, even though he still goes to a standard public grade school.
If Dexter has a main weakness, it's that his intricate plans are somehow foiled by the lesser intelligent, usually in the form of his older sister Deedee.
Stewie Griffin is a 1-year-old baby with a single goal: Total World Domination. He has the voice and manner of an evil Rex Harrison, but he's only recently celebrated the one-year anniversary of his escape from his mother's "cursed ovarian Bastille," in which he was incarcerated for nine gruelling months. Stewie has vowed to defeat his mother's matriarchal tyranny and topple the "gynocracy" she rules.
Just because Lois has narrowly escaped several attempts on her life thus far (from a box of chocolates filled with active grenades to a barrage of arrows shot straight for her head) doesn't mean she's in the clear yet. Stewie might begrudge Lois a modicum of respect for being the worthy adversary she is, but don't be fooled. One day her uppance shall come!
In fact, if it were not for his lack of muscle tone, toilet training and his need for parentally provided sustenance, Stewie would have already gained control over most of the third world, including Canada. Until his dominion over all mankind comes to fruition, anyone or anything that interferes with his grand plan shall be destroyed. And if he can create a machine that controls the weather, what makes you think he won't be able to control you, hmmm?