I hope this is not the way that Nintendo will follow, although that's the easiest way out, because in my opinion they were not designed and indeed they were not presented as adopted children. Three of them even have blue eyes like Bowser had in SMB3. I really dislike the adoption theory. Luckily, Nintendo UK agrees with me:Iggy Waffles said:If they were adopted it would make more sense. Since they were disowned as Bowser's actual kids.
https://www.nintendo.co.uk/Games/NES/Super-Mario-Bros-3-279558.html
"And youll need all the powers you can muster, to beat Bowsers offspring lurking in their heavily-armed airships."
P.S.: of course, also Nintendo of Germany used that term:
"Du wirst all deine Kräfte benötigen, um Bowsers Sprösslinge zu schlagen, die in ihren schwer bewaffneten Luftschiffen lauern."
Germany loves the Koopalings as Bowser's natural children...
and Nintendo Italia agrees!
"Per battere la prole di Bowser, asserragliata su navi volanti pesantemente armate, ti serviranno tutti i poteri che riuscirai a raccogliere."