Sometimes after trips like that, young kids have a bit of an emotional let-down. All week, or however long the parents were away, the kids are great and held things together on the surface OK, but inside they did have emotions building up... maybe they are upset about their parents leaving them, or they miss them, or they feel angry... they don't know how to talk about these things really and they also want to please you by being happy... when their parents appear again... the floodgates open. They can relax, and let go of all their pent-up stuff!
It'll pass.