Jean-Luc: What do you think?
Carol Foster Lambert: I think you look like Barbara Eden from "I Dream Of Jeannie."
Jean-Luc: Well, I think you look like Crissy from "Three's Company".
Cody Lambert: Woah! A Canadian penny! Now I can buy bubble gum! That is, if I go to Canadia.
Gabrielle, Mark's girlfriend: Be still, my heart.
J.T: Be still, my lunch.
Answer: According to the SBS website, Brendan's disappearance was never really explained. He just kind of faded out. He was hired as the cute "little" one and as he got older he wasn't as cute. When the Lamberts had the little girl, they got their cuteness factor back and didn't need as many kids around.
Carol and Frank continued to tell people they had seven children throughout the remainder of the show's run, but they never referred to Brendan after season 6.