This is the 1745th most frequent Russian word.
детские
"Children's" or "childish," depending on context.
In this sentence, the word "детские" functions as an adjective, modifying "игрушки" to indicate that the toys belong to or are associated with children.
Детские игрушки лежали на полу.
Children's toys were lying on the floor.
Here, "детские" describes "книги," referring to books intended for or related to children.
Мы купили новые детские книги.
We bought new children's books.
In this example, "детские" modifies "воспоминания," meaning the memories pertain to one's childhood.