This is the 163rd most frequent Czech word.
dětí
The Czech word "dětí" means "children" (genitive or plural form).
Here 'dětí' is used as a genitive plural of 'dítě' (child) in a context discussing books about children.
Mám rád knihy o dětích.
I like books about children.
In this sentence, 'dětí' is the genitive plural form and is used to denote possession, showing that the play belongs to the children.
Hra dětí na hřišti byla veselá.
The children's play in the playground was joyful.
'Dětí' here also serves as a genitive plural, used with the preposition 'o' to describe a quality attributed to children.