What is even more shocking is that Chomper never even mentions what type his friends are (they could be fellow Sharpteeth for all they knew), yet they still said they wanted them to be dessert. I was actually most shocked at that reply of Chomper's parents and wonder why they didn't try and eat Ruby.
Excellent observation!
I don't think anyone has ever pointed out that one, but indeed that is remarkeable.
About the premisses for the start of the TV series we can only speculate. LBT V suggests that the island is problematic (not much to eat) for the sharpteeth, so they would probably have to leave before long. This would either suggest another natural disaster opening up the way to the island at least temporarilly (had that one as withdrawing water prior to a tsunami in one fanfiction) or a more complex plot allowing for at least Chomper to leave the island. It seems unlikely that he would leave his parents to starve, but maybe they suffered a different fate? Ruby says that she promissed to Chomper's parents that she would take care of Chomper; but why would they want her to take care of him?
There seems to be a certain probability that Chomper's parents are no longer alive by the time of the TV series. If one further speculated on a somewhat darker level, one could wonder if in case Chomper's parents are really dead Redclaw might have something to do with it (which would mark him as more than the everyday kind of sharptooth the gang is dealing with all the time). These are at least some thoughts I have in mind for a story I have never penned down though I did draw a few images based on it.
As to that, I believe that is much less sinister. Likely Chomper did something to offend Red Claw and had to run for it. I doubt his parents are dead.
As for why Ruby could return to her family, my theory is that she is NOT Red Claw's main target, but only got herself drawn into this by befriending Chomper. If she left him, she could probably wander freely. They make it quite clear that CHOMPER is Red Claw's target, and thus, he'd have to run. Being with his parents would put them all in danger. Ruby seems to have agreed to protect Chomper. Also, the fact that Ruby would, at first, want to possibly go alone to Hanging Rock indicates that she is NOT the target of Red Claw, as she would, hopefully, have enough sense not to risk her life to do such a thing, knowing her parents would NEVER want to run the risk of her being a meal for Red Claw just to come see them.
Another thing is that there is one Red Claw and one Screech and Thud. If there is only three of them in total, then why couldn't Chomper's parents, plus Ruby's parents, plus Chomper and Ruby be able to take them out alone? Perhaps Red Claw has a whole league of minions, and Screech and Thud just happen to be his bodyguards or something
As for the starvation danger, I suspect that is passed as years have gone by, no doubt, since the fifth movie events. If they didn't die of hunger earlier, they probably aren't in danger now.
While losing his parents to a crazy Sharptooth psychopath would enable Chomper and Littlefoot to bond better due to mutual experiences, I doubt that is the case.
Also, if Screech, Thud, and Red Claw HAD indeed killed Chomper's parents, one would have surmised that Chomper would have let Thud stay trapped there and starve to death as payback.
As for the island, they may no longer be on it. There may be another path off of it or something. Chomper and his parents couldn't have explored the island fully as they had no idea the plated Sharptooth was there. (Also, considering that the parents COULD have left, regardless, until the tsunami, they needn't have just looked for food on the island.

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