In 1983, DIC, a French-Canadian animation studio, made its first inroads in the United States. The studio acquired the rights to Heathcliff, who'd previously spent 2 seasons on ABC, and developed Inspector Gadget, both for weekday syndication. Their 3rd entry, airing on ABC, was The Littles, based on a series of children's books by John Peterson.
The Little family lived within the walls of the Bigg household, and befriended Henry, who kept their existence a secret from his parents. In a way, this was similar to another book, The Borrowers, which was adapted for television in the 70's, and became a feature film just a few years ago with John Goodman. Here's the open for the first season:
The Littles were meant to be ABC's answer to Smurfs, over on NBC, but lacked the staying power, airing only 3 seasons. It's been revived in syndication over the last few years, but I believe its cable rights now belong to the This network. A made-for-TV movie, "Liberty & the Littles", produced in 1986, aired on ABC's Weekend Special, serving as the coda for the series.