Learn German: Die Kinder spielen drinnen. - The children are playing indoors.

Words with hin and her show movement toward a destination (like running into a room). In this sentence, the children aren't moving from the outside to the inside; they are already playing in a fixed location. Because there is no change of location, we use the location word drinnen (indoors/inside).

Can I just say Die Kinder spielen in instead of using drinnen?

No. The word in is a preposition, which means it needs a specific object to point to—like in dem Haus (in the house). If you just want to say "inside" or "indoors" all by itself without naming the building, you must use the standalone adverb drinnen.

What is the opposite of drinnen if they were playing outside?

The opposite is draußen (outdoors/outside). Just like drinnen, it describes a fixed location rather than a direction of movement.

I always mix up singular and plural for "child". What are the clues in this sentence?

Two things tell you this is plural. First, the noun is Kinder with the plural article die (instead of the singular das Kind). Second, the verb spielen (are playing) has the plural -en ending, rather than the singular spielt (is playing).