This is the 140th most frequent Japanese word.
子ども
Child.
Here, 子ども refers to a specific child who is actively involved in the action of playing at the park.
子どもが公園で遊んでいます。
The child is playing in the park.
In this sentence, 子ども is used in a general sense to indicate all children as a subject of the speaker's affection.
私は子どもが大好きです。
I love children.
This instance of 子ども signifies a specific child belonging to the subjects 'they' (彼ら).