Mrs. Murry knows that Meg and Charles Wallace are very smart because she and Mr. Murry gave them IQ tests before Mr. Murry disappeared. This is important for Meg to know: the local community thinks she and Charles Wallace are "dumb" because they are different.
Meg tells her father that she doesn't want Charles Wallace to end up "dumb like me." She also wonders, when her father assures her she is smart, if he thinks so just because he loves her. He assures that is not so and that the parents have tested her. She then remembers games her parents played with her that must have been IQ tests.
However, when she asks her father for specifics about what her IQ is, he won't tell her. He does let her know that she and Charles Wallace are smart enough to "do pretty much whatever you like when you grow up."
Because Charles Wallace has blossomed since her father disappeared, Meg believes what her father said.