The Secret Throne - Peter F. Hamilton

The Secret Throne is the first ever children's fantasy title from sci-fi author Peter F. Hamilton.
Taggie and her younger sister Jemima are just like any other girls their age. They live with their mum, go to school, play sports and spend their holidays with their dad in the countryside. But one day, everything changes ... when a white squirrel wearing purple glasses turns up in their lives.
The next thing the sisters know, their dad has been kidnapped, and it's up to the girls to get him back. But it seems their father has been keeping some very large secrets from his young daughters, for their dad is no ordinary man. He is an otherworldly prince in exile, and the land he should be ruling, the First Realm, has been overthrown by the King of Night.
Can Taggie and Jemima find their way between the worlds to save their father, and what other secrets has he been hiding? Before long, the sisters realise that it is not just their dad's life at stake. It's the future of a whole world.