This was a very fun and enlightening story. I loved Chomper and his antics in this story and it took real effort to make Elsie seem like a good character. I found her annoying in the film abut she truly works here. Well done on that.
The characterizations were excellent throughout. Chomper's parents' personalities seemed quite natural and their dialogue was believable. However, the conversations about "El Zee" truly excelled with their hilarious dialogue and the characters' antics.

Chomper's insistence on her existence and his parents' attempts to reason with him were priceless. I know it sounds strange but I seldom laugh when I read anything but here I chuckled throughout. Great job!
Elsie herself seemed like an interesting character, one that could have actually improved the film. Your descriptions about her past managed to build enough background for her to be interesting and her drive to avoid sadness is something I can easily relate to. Moreover, her character was very likable but not unbelievably so. The slow collapse of the island was a nice thing to add as it could at least partially explain the extremely realistic pathway to the island.
I especially enjoyed the lively dialogue, great descriptions and the look to Chomper's early childhood. This truly managed to deepen the lore behind the fifth film in an entertaining and imaginative way. This was an amazing story and it truly brightened up my day.
P.S. On a side note, I wonder if it's necessary to come up with original names for lore characters in every story? Your names for Chomper's parents worked well but sometimes it gets somewhat confusing when every story uses different names for the Gang's parents', for example. I don't know what can be done about it, though.