From Gamehiker Wiki
The Koopalings are seven young Koopas serving under King Bowser Koopa who first appeared in Super Mario Bros. 3. They made frequent appearances as bosses in games made during the early 90's, but they fell into obscurity until they finally reappeared years later in Mario & Luigi: Superstar Saga. They have returned in New Super Mario Bros. Wii, with all but Wendy receiving an upgraded appearance.
They were originally known as being Bowser's children, but in a 2012 interview, Shigeru Miyamoto clarified that none of the Koopalings are actually related to Bowser. For Bowser's only actual child (previously assumed to be the eighth child), see Bowser Jr., and for his similarly-titled miniature dopplegangers, see Koopa Kid.