They could do it as a flashback, with the flashback being the whole episode. They could do it as perhaps 2 episodes, with Chomper's story of how he got off the island, met Ruby's family, how Ruby came to promise his parents to look after him and how they got the great valley grown ups to not mind Chomper being there, in one and more of Chomper with Ruby & Ruby's story in another. Though they may decide to keep Ruby with Chomper living in the Great Valley as a tv series only thing and keep the movies as happening before the tv series.