What's the Name of That Book??? discussion

This topic is about
The Wind Singer
SOLVED: Children's/YA
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SOLVED. Missing part from giant metal songbird needed to destroy old children YA fiction, adventure, dystopian. [s]
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The book is centred around one family, and the entire city is ordered in classes ranked in colour, purple the top dogs. Every year or every couple of years everyone is ranked and either stay in their colour or moves. This family I believe were orange and moves to grey after the baby pees on the dude doing the assessment.
There is a mum and dad, a brother and sister. (I think they were twins?) I can't remember the brother's name but I think the sister's name was Kestral? I think their cousin also lives with them, who has some mental disabilities and his name is Mumper. The baby's name starts with P, something like pumpkin or pimpkin.
Anyway, they live in this city and in like one of the great halls or something there is this giant metal bird that no longer sings because it is missing something from the pipes. The urban legend is that is this missing part is found then the city is free from the outside bad guys. The villains are called old children, who are like children but old and if they touch you, you turn into one of them. I think the sister discovers them in classrooms in the city (locked up) and this is what causes her, her brother to leave, and I think Mumper follows them, I can't quite remember. I think Kestral discovers the actual king of the city in a tower during this part, who seems kind of senile, but kind of there. But he is obsessed with chocolate buttons and this is why he doesn't leave his room because he will run out of chocolate buttons.
Anyway, Kestral and her brother leave the city because Kestral has seen something she shouldn't have, and they are searching for this missing part of the songbird in the hopes that they can fix their problems.
At some point in their journey, they are being chased by these old children and Mumper gets caught by one and I think he disappears for a bit. Anyway, they get picked up by these fighting landmass ships (Along the lines of the Mortal Instruments) and are put into jail. Because of their ill preparation for this long journey they are kind of dying. Their guard said something along the lines of "if you die, ill kill you" because they are prisoners of some sort? I remember this line sticking out because well, it seemed funny. At some point, they get off this ship and find the missing part that they need to bring the songbird back to life. It is attached to a sleeping old child. The brother removes the piece which she is wearing like a headband and snaps a bit of her hair which causes her to wake up (I'm not too sure what happens in this part) . They head back to their city, being chased by these old children, fight their way in and managed to get into the hall where they put the missing part back into the songbird who starts to sing and the old children disintegrate (?). Also, the way the city is set up in class ranking is also dismantled.
Now there are some of the details I'm not so sure on, I can't quite remember if they get picked by the ships before or after they get the missing part.
I think the book title is along the lines of the song of the wind bird or the wind birds song but I've tried googling various versions with this without much luck. The author I'm pretty sure is a man if this helps.
This book has been on my mind for the past couple of years, and I would really like to read it again so I hope I have given enough detail for someone to know what I am talking about.