Hi. Some parts of your answer reminded me of a book I used to love called 'The Midnight Kittens' by Dodie Smith. However it was an orphaned brother and sister who lived with their gran and one night when they put out bread and milk for a hedgehog,four little kittens appeared and the children thought they were magical because they always appeared at midnight.
Obviously that doesn't all tie in with the details you remermber so I may be totally off track here. Good luck tracking it down. x