I'm a fairly new writer, and I've been thinking about a story. It'll be based around a pair of characters, both children. One's around twelve and the other sixteen. The latter was recently adopted into the former's family. The two were just getting to know each other. To make everything more interesting, I've decided that the twelve year old would be especially violent and cruel while the sixteen year old would be pacifistic. Basically, it'll go like a standard fantasy story. They'll go to some fantasy themed place and try to find their way home. But everything will be figurative-you can't really decide if their adventure actually happened or if it was just something made up by a child's imagination. I might also play around with the story a bit and make it slightly darker, with things like depression. What are some ways I can make the interaction between these two characters believable while developing their relationship? And what are some things to avoid while writing this?