The Ocean at the End of the Lane by Neil Gaiman.
Gaiman is a master at creating magical stories and this novel is no exception. In this sad, but enchanting story there are "fleas," "varmints," and a diabolical babysitter.
A "flea" is a monster who embeds a path into its victims that allows it to escape from its own world. Though the boy does not realize it, imprinted on his heart is a portal. The "flea" uses him to gain access to him and his family, nearly destroying them in the process. A "varmint" is a large bird-like creature that is essentially a scavenger.
George puts his trust in Lettie and her pond, which she calls an ocean. Together they battle the "flea" and the "varmints" that hunt them. Will Lettie's magic and the healing powers of the ocean be enough to save them?