This is the 78th most frequent German word.
beiden
"Beiden" means "both" in English.
Here, 'beiden' is used as a determiner to refer to the two children specifically.
Die beiden Kinder spielen im Garten.
The two children are playing in the garden.
In this context, 'beiden' is acting as a pronoun referring to the two students.
Ich habe beiden Schülern die Bücher gegeben.
I gave the books to both students.
Here, 'beiden' is modifying 'Städten' (cities) to specify the two involved.