The Seven Little Monsters have a father, however he is never actually seen on screen. There was an episode where Mom told Two that he reminded her so much of his dad.
Even though Daddy is never seen in the show, there is in one episode "A Clean Sweep" when the monsters heard a noise coming from their mother's room, a photo can be seen behind Mom of her and her husband. The photo is seen in another episode but it's a little fuzzy to make out.
There is no explanation of Daddy in the show, nor is it ever explained if he is alive or dead, so it is a mystery of where he is. However, in ''Good Morning'' and ''The Mystery of the Missing Five'' she says of how Two and his nose reminds her Daddy.