when do the snapchat stories take place, because I always thought they were after the credits, but if the beta kids get sucked into the juju then how can they take part?

They take place after the Credits, and, if the Masterpiece happens in the same Timeline, before they go fight Caliborn. Again, IF, there’s hints that the Masterpiece Timeline may not be the same Timeline of the Credits, namely Caliborn seeming surprised even though he’s literally watching it happen on the screens.