This is the 2891st most frequent Polish word.
małej
"Małej" means "small" or "little" in the feminine singular genitive, dative, locative, or singular accusative (when describing animate nouns).
This usage shows 'małej' as a possessive descriptor (genitive case) referring to 'dziewczynki'.
Widzę małej dziewczynki kotka.
I see a kitten of the little girl.
Here, 'małej' is used in the dative case referring to 'uczennicy', describing that the book is given to her.
Daj książkę małej uczennicy.
Give the book to the little student (female).
In this sentence, 'małej' functions as an adjective in locative case relating to 'rzece', specifying location.