The Koopa Kids are minions of Bowser who only appear in the Mario Party series. They are known as Mini-Bowsers in PAL territories, Mini Koopa in Japan (Bowser's name in Japan is Koopa) and Baby Bowsers in Mario Parties 1, 2, and 3. The Koopa Kids resemble miniature versions of Bowser (and are therefore very similar in appearance to Bowser Jr. and Baby Bowser). A Koopa Kid also appears as a playable character in Mario Party 5 and Mario Party 6.
In the earlier games in the series, there appears to be an arbitrarily large number of Koopa Kids, as shown by minigames such as the Mario Party 3 mini-game Baby Bowser Bonkers. Indeed, Mario Party 4 specifically refers to the Koopa Kid who appears on Bowser Spaces as "one of the Koopa Kids". However, starting in Mario Party 5, there seems to be only a single Koopa Kid, likely due to his new playable status. However, more than one can appear at once in Mario Party 7 (where Koopa Kid is non-playable again) in Pyramid Park.
Mario Party 5 also introduced Green K. Kid, Red K. Kid and Blue K. Kid (Green M. Bowser, Red M. Bowser and Blue M. Bowser in PAL versions) who, as their names imply, are colored differently. In Mario Party 5, their first appearance, they are shown to be the result of one Koopa Kid splitting into three.
In Mario Party 7, Koopa Kid is no longer playable. He has his own orb that Bowser sometimes sells in his shops. When it is obtained, it will add a Koopa Kid Space to the board.
Koopa Kids have seemingly been removed from the Mario Party series starting in Mario Party 8, likely in favor of Bowser Jr., who appears in Mario Party DS onward.