You might be a little confused with the title since they aren't actually parents, but it's weird since they are.
In the Baby Spencer videos, they act like Baby Spencer's parents.
In The Jitters, they actually are parents. They're married and have Spencer and Gibby as their kids.
In iSpace Out, the mystery girl looked like Sam and Freddie's future kid.
EDIT (Thanks to IgnaLovesPancakes) : In iLook Alike, Sam and Freddie were Carly's parents in the caveman sketch.
Is there anything else I'm missing?