The Fabulous Zoo. Rag & Boyd, #1

Following the disappearance of their father and sudden death of their mother, Morag & Boyd must go and live with their parents' childhood guardian, Wulfric Kennetson. The reclusive Uncle Wulf, it turns out, owns a remote castle and estate in the far North West of Scotland, but this is the least strange thing about him. His staff are all unusually short, and his two large dogs have dark green fur. The animal sanctuary he runs is home to numerous rare species.
And by 'rare' that sometimes means 'previously only known in fantastical tales'. Discovering clues to their father's whereabouts, Rag & Boyd realise that he is in a different world, beyond the castle's secret doorways, and he may be in deadly danger.