What is a kid friendly definition of subplot?

Answer 1

Also called storyline. The plan, scheme, or main story of a literary or dramatic work, as a play, novel, or short story. plot is the sequence of events or actions in a short story, novel, play, or narrative poem

Answer 2

The main plot is the most important story that is going on. But sometimes the storyteller also tells you about another story that is going on at the same time, but isn't as important as the main story. This less important story is called a subplot.