Ok. here is my explanation. THere's a bunch of types small Bowsers.
Baby Bowser: This is Bowser, only as a baby. He is the Bowser equivalent to Baby Mario. He appeared in past games such as "Yoshi's Story" and "Yoshi's Island."
Koopalings: The Koopalings are in fact Bowser's direct children. Although they disappeared for quite some time, they are still existent as proved in Mario & Luigi: Superstar Saga.
Koopa Kids: These aren't necessarily children of Bowser, but are probably just relatives of him, hence the name Koopa Kids. They do show a striking resemblence to Baby Bowser, however, in no game were they said to be Bowser's Children. They appeared in the Mario Party games but no where else...
Bowser Jr.: The most recent addition to the Bowser family, he is in fact Bowser's son. Due to his distinct Bowser looks, and name, he is obviously Bowser's son. Since the Koopalings don't appear to be related to him much, it can be hypothesized that they have a different mother than Bowser Jr.
I am Meowrik! The 5th Samurai Pizza Cat! beware the fearsome fury of my fiery Jalapeno Katana!