I loved The Littles as a child. I never read The Borrowers by Mary Norton, although it has been around longer and is a more complex and well written series, but I gather that the main similarity is that both the Littles and the Borrowers borrow things from humans and repay them in the best ways they know how. Of course, the huge difference between these books is the length and reading levels.
Begun in 1967, The Littles, with more than ten books in the series, uses plain language and simple descriptions to tell the story of this family and their adventures inside and outside of the house they inhabit. The interior illustrations are well done and capture the liveliness of the different characters. These books should appeal to both boys and girls and are a great alternative to the two very popular, powerhouse series written at this reading level - The Magic Tree House and Junie B Jones.
If your child likes The Littles, suggest The Borrowers, RL5 and the Bromeliad Trilogy, RL5.