Learn Russian: Ребята играют в парке.

Both words mean children or kids, but they have a slightly different feel. Дети is the direct, standard plural of child. Ребята is more informal, much like saying the kids or the guys in English. It is very common in everyday speech when talking about a group of children playing together.

How do we know играют means are playing and not just play?

Context tells us! Russian does not have a separate continuous tense like English does. The single present tense verb играют covers both general routines (they play) and actions happening right this second (they are playing).

Why does the word парке end with an -е?

Because it is in the Prepositional case. When you use the preposition в to talk about a static location (where something is currently happening), a regular masculine noun like парк changes its ending to an -е.