Simile is used when the author wants to simply make a comparison. They use like or as to do this. “Red as a rose” is often used to describe a woman’s lips, while “strong as an ox” is a good quality for a prince to have. A simile describes a trait that someone has, and is usually given as an image that can be taken literally.
A metaphor, on the other hand, means that something is standing in for something or someone else. “A lion’s heart,” for example, means that person is very brave. They don’t actually have a lion’s heart. If you say there’s an army of children, unless this is a military thriller, there’s not an actual army of children; there’s a lot of them!
Both are used in literary writing but rather differently. Simile is when you find one thing like another so you compare them. For instance, the light in a lover's eyes may be "like a moonlight reflection on a pool". A metaphor is using one thing to stand for another.
"An army of children" would be a large number, probably lined up. The reader would not be expected to think of them as an actual army. Instead of using "like" the above may be written as 'His lover's eyes were a moonlight reflection on a pool'