It isn't explained explicitly. I theorized in post # 16:
"If I recall correctly it has been previously established that Neal/Baelfire was in Never Land as a child. How long we don't know. And they seemed to have established in this episode that there is a feud/history between Rumple and Peter Pan/Never Land. I wonder if Rumple ever knew his son spent time here.... If this is a place (or a person in this place) he truly fears perhaps it upset him to know his son spent time here? Or just being reminded of such, if he already knew (reminding of his ultimate crime of abandoning his son)?"
Or there could be another story to it.
We just met them, as did all the characters except Hook (and possibly Rumple, but he wasn't present in those scenes). So watching this episode you know as much as we do.
Fast enough with the questions? You do realize none of the main characters actually seem to know the answers to these questions and we just got to Never Land? I thought these questions were meant for us, but perhaps you mean them to go out into the ether?
What we did see in this episode was that Peter Pan (and or the shadow?) wanted the heart of the Truest Believer. That would make sense that it would have strong magical powers. With or without the rest of the body. Perhaps he needs Henry to do something, or perhaps he just needs his heart. Perhaps Peter Pan is sick of Never Land and can't escape himself (though how they got those two idiots they killed involved would perhaps be a bit harder to explain). Perhaps he wants to defeat the dark shadow. Or Hook. Or some other island creature/person. Perhaps it somehow involves Henry's dad Neal/Balefire (I can't imagine he left Never Land with the blessing of the Lost Boys or the shadow.)
Do you know that Henry's biological father is Neal/Baelfire? And Neal/Baelfire is Gold/Rumple's son? So Henry is Gold/Rumple's grandson. And Gold/Rumple (and everyone else) seems to presume Baelfire is dead, because he was severely injured and fell thru a portal to where they don't know. Though there is some twisted prophesy that Henry is going to somehow cause Gold/Rumple's death the Gold/Rumple got before , the curse. The prophesy didn't mention their relation. So why does Gold/Rumple want Henry? To save him? Or to sacrifice him to save his own ass? Or something else? (maybe he'd like the magic that comes with the heart of the truest believer)
Finally! A question I can answer!
Baelfire fell into a portal, when mortally injured, as I mentioned before. It was not on purpose, but apparently he has been thru a few and knows how to work them (I'd guess from his comment about the last thing he thought of was Storeybrooke). As you could see he was surprised that Emma is in Never Land. He fell thru the portal before Henry got kidnapped and the rest followed.
Robin is convenient. Storeybrooke is pretty depopulated since the curse. As Robin mentioned, he owes Rumple. Once Rumple was going to kill Robin. I can't remember the episode well. I think Robin stole something to save an ill pregnant Marian or something (I am wondering where she is in all this). His purpose may just be being a sidekick who knows the new Enchanted Forest We'll find out if it is something more.
Snow (and Charming) want Emma and their grandson Henry. They wouldn't leave Emma to do this alone silly