"The little girl often jumps in the house."
Translation:La malgranda knabino ofte saltas en la domo.
-et is the suffix for something small, whence eta, as an adjective, just means ‘small’ and is pretty much synonymous with malgranda.
So if you changed "domo" to "domon" would it be, "The girl often jumps into the house"?
Not if they stand right in front of an open front door and make a tiny hop forwards ;).