Could it have been The White Rabbit's Road , something I've been trying to get my paws on ever since I read it as a child? The white rabbit is the Queen of Elfland's messenger and unspecified hostile faery forces are trying to stop him getting back to Faery, and there are three possible routes he can take. And there's a deadline; sunset or dawn or something.

He engages the children to help him.

Alternatively, I came across a series of books with children passing into and out of fairyland which were set on a mythologised form of the Scilly Isles.