Possible spoilers for season 6 of OUAT, but no knowledge of anything beyond this plot (unless playing a canon) is needed.   

Once upon a time, there was an enchanted forest filled with all the classic characters we know.  Or think we know.  One day they found themselves trapped in a place where all their happy endings were stolen.  Our World.  This is how it happened...

Before the Black Fairy, King Arthur, and all of the other complications in the lives of the fairy tale characters we know and love, back in the time before Emma broke the curse... This is where our story begins.  The residents of the Enchanted Forest and the various other nearby lands were swept up by the Evil Queen's curse.  

This may be the beginning, but their story has already been told.  And it failed, leaving an evil fairy in charge of the world with the worlds of story destroyed.  However, a few of our heroes who survived, Henry, Gideon (who his parents had given his life to retrieve his heart before this spell), Violet, and Baby Charming, have gone back in time to change history.  They just landed a little bit too far back.  

Now these next gen children are in this world, hoping to save it from the Black Fairy and the grim future they had come from.  Of course all magic comes with a price, and uncertainties, so these travelers have landed in different times (essentially meaning they arrive when they are created, no matter where we are in the plot), and must make sure that they do not interfere in the future too much, as they could end up changing things into an unpredictable future they can do nothing about.  

The little town of Storybrooke is currently still cursed and Emma has yet to arrive (though when a player who plays Emma comes, that's when she arrives, similar to the next-gen travelling kids).  Regina is the only one who remembers at this time (unless you have a good reason for another to be woken).