Thief of Time discussion

Hi, so I am looking for a lost book. I read this a long time ago, well ten years maybe. But it has a yellow or purple cover, and is about two or three kids. They are on their summer vacation, and then somehow, maybe take a train to a mountain, where they enter a cave which takes them to this new world type thing where mythical creatures lived. They had some sort of guide, who could be a bird. There are unicorns. Cows, and have snail shells. On their shells there are faucets for cream and milk. Then when the goo from their sliding shell (like a snail) dries it's super useful. There were also cork trees, and bottle trees. Some sort of talking ferret, or similar animal, fire breathing roosters, a dragon that lives in another cave. The children come to save this place, so they travel around to enlist help. then at the end there is a siege on some sort of castle because they are trying to help their guide. do you recognize it?

This is the wrong place for this question, you aren't any longer activ on GR, anyway I'll give it a try: I'm pretty sure the book you read was 'the talking parcel' (also called The Battle for Castle Cockatrice) by Gerald Durrell https://www.goodreads.com/book/show/4...

I'm so glad this mystery was solved. We can all rest easier.