While Baby Mario and Luigi have their own series, they are sub series characters and Bowser is tied to the Main series that features both him and the Mario Bros as adults. This is where the Koopalings got revived in a sense and then subsequently retconned from Bowser's family tree.
Remember the first batch of Koopalings? When Super Mario world was first released and each Castle was guarded by one of his many children, they were used sparsely since then and sort of left dormant until the realized that a version of Baby Bowser would seem more apt for introducing a fresh antagaonist into the Mario Universe as opposed to reusing the already zany and not that gripping set they did before.
This brought about what's known as the
Koopa Kid(s), which is basically a condensed form of Baby Bowser that can exist in the Main series without having to bring Baby Bowser into the universe storywise and cloud things. They were basically minature versions of Bowser that looked like his own younger form, often with pallete swaps since they were mainly used in the Mario Party series and other gaming spinoffs.
Bowser Jr, per Nintendo's insistance, is the only Offspring of Bowser after they decided they wanted to go full on with creating a cuter, more comedic foil for Mario to battle since Bowser is often only shown as Gruff and domineering and Nintendo started to make games in which Mario was called to fix problems created of impish foil (Mario Sunshine where Baby Bowser turned everyone to Goo and Mario Sunshine where he scattered the stars'n'shyt) that they felt lent better to the overall lighthearted tone of the current games they were making as opposed to just sending Mario out, yet again, to go fight Bowser.
Appearance Wise, they aren't hard to spot if you can tell the subtle but definitive diferences between them all.
Bowser and Bowser Jr look the closest to each other, but Bowser Jr is almost never with Bowser since it's basically the same character within the same time line, which wouldn't work. So if you See a Big Bowser and a smaller one, that's more than likely Bowser Jr, who is always shown with a Bib that has sharp teeth. Baby Bowser never wears a bib.
Koopa Kid/Mini Bowsers are just smaller versions of Bowser that are his hench men and often never take the identity of either or any overt authority outside of tormenting the Mario Bros on the behalf of Bowser's empire.
Dunno if you or anyone else read all of this, but if you did and it helped, then cheers.