"The Midnight Kittens" 
by Dodie Smith

I'm sure you all know "101 Dalmatians" by this author- the book and the brilliant film. This is less well known, but worth a look, especially if you are a cat lover.

Orphaned twins Tom and Pam live with their grandmother. They discover the mysterious kittens coming out of a tree at midnight and drinking the milk left out for hedgehogs. This is the beginning of an adventure involving secret rooms, squatters and a touch of magic....