Translation:The girl with green eyes is my cousin.
Out of curiosity, is "...se zelenými očimi..." also correct? My classmates preferred to use forms like "s náma" and "s váma" but in written form I saw "s námi" and "s vámi" more often.
No, that is the point of this lesson. Unfortunately we still do not have the Tips and Notes.
The rule is: "Paired organs of the human body (ruce, nohy, oči, uši, ramena, kolena) use even the so-called dual declension in the plural (you can find more about the declension of body parts here). " from https://mluvtecesky.net/en/grammar/cases
So you have očima as the correct form, but you have s námi for other plurals.