
bro, letting kouichi and klaus keep kluluk wasn't that hard, you ALREADY have a trained prince whom was promised the throne for 2 decades. BFFR!!!!! that kid didn't wanna go, and I'm sure he'll runaway

the decline came for the abrupt change of environment inside the castle.
He was a kid whom childhood was stippred away within seconds of being born, grew up been call "the future and ONLY king", gave up on his own desires/dreams/infance to live up to OTHERS expectations about him. Just for his dad to fall in love with a person young enough to be his sister, get her pregnant AND throw him out the windows to make space for the new child.
It's better if the people at the castle FIRST help him go back to his old self, SECOND the 2 sibling have a chat with the dad, AND LAST explain him the situation with the new baby. The trained prince entire life was built around becoming the king, so he needs time and patience to adjust to his new reality if the bird people wants the NEWBORN to be the king now.
Also, Kluluk doesn't seem to want to be away for kouichi and klaus, so we'll see if he ends up running away or giving up the throne to be with them.

we all know the situation here, that baby can't stay in with the bird people.
1) the castle environment will be harmful for his development
2) you already have ONE capable and trained prince to ascend to the throne
3) he has an emotional link to klaus and kouichi, tearing them apart could made hin enter a state of shock
4) have you ever of adoption???
as far as I remember from an old flashback shown, seems to be our MC is the reincarnation of someone the mugo's met before they became that entity. Now, I don't trust "sir mugo" the "kind" one the way he treated the little ones was weird, he said they bring bad luck... to who??? him?? why??? what has happened before that they triggered him in a bad way?
Someone said far back, the gummies could be the ghosts of past sacrifices. So maybe it's bad luck for him. If the gummies could speak well I think they'd have told mC a lot. (⊙…⊙ )
that makes sense, because they DO speak but in some sort of sound only, so if mc finds a way to translate them, it could mean problems
And maybe it's just me, but the gummies seemed more hostile with sir mugo than the other two. So, something's really up with him.
yeah!!! and maybe that's why he just vanished them, because other mugos seem ANNOYED but this one was MAD MAD as if something will go against him