The Earth Children and The Golden Key

Tania and Jal are siblings who live with their parents just outside of London. They are an ordinary family and attend an ordinary school, a couple of blocks from their house.
Tania is 10 and in Year 6 and her brother is a year younger. They get on well, but Tania is a lot more sensible than her brother. One wet, blustery day, the children discover a secret opening to a hidden city in the sky. They meet a bird called Corin who takes them to this city, which is hidden in the clouds. The children meet Hannah, a child who lives in the clouds and whose parents have been kidnapped by the evil Borro. They also learn that Corin's wife and children have been taken and agree to help the bird and Hannah find their families.
With the help of 4 powerful Guardians, the children travel to Borro's land. They travel on a a cloud ship in order to reach their destination. Their journey is fraught with danger but they eventually make it to Borro's foul-smelling city, where they believe the lost families are being held.
Will Tania and Jal succeed in helping Corin and Hannah find their families? Will they defeat Borro and help to restore harmony to the tumultuous sky?