The movies very much dodge the point and the makers of the movies can afford to do so as the gang was never ever going to grow up. There is one Dinotopia novel by Donald Glut about pretty much the same problem. Funnily enough, that novel is named after its main protagonist, "Chomper" (I guess it is more likely than not that the author of the book (published in 2000) may have taken a little inspiration from the land before time)
In that book "Chomper" ultimately reaches an age where he cannot always control his instincts anymore and where some events may trigger his chasing instinct. He gets very close to actually attack the one who hatched and nurtured him. After that he decides to go to the rainy basin but he and his friends from the Dinotopian civilization continue to meet on regular basis.
I would at any time guess that if given the choice Chomper (LBT Chomper now) would rather gnaw of his own arm than attack any of his friends if he has the choice. I do think however that instincts could chime in and I also think that if irritated and furious he might loose control in certain situations and not always be able to predict both the effect of his claws or teeth and the effect which even a minor scratch would have on his reputation in the Valley.
I don't know if there would be in the long run sufficient fish in the rivers of the Great Valley for Chomper to live on them exclusively. Also notice that T-Rexs in LBT are not presented as particularly aquaphile so it might be problematic to turn Chomper into a fisher.
As for scavenging it would make sense to do this the Dinotopia style, but this is another matter which land before time has been dodging so far (even though scavengers could deliver plenty of material for more than one story). Of course there would have to be a certain constancy in the regular death rate of the Great Valley to ensure Chomper always has enough. But honestly, imagine this:
Littlefoot: My grandpa died!
Chomper: I'm very sorry Littlefoot.
... uhm may I?
Littlefoot: Help yourself
I don't think so
While it might theoretically be possible to feed Chomper by fish and carcasses this would not necessarily help against the aforementioned instincts which might or might not come up for Chomper (note that the Chomper in the Dinotopia book is quite sufficiently fed with fish at any time so the instincts are the only thing that makes his leaving necessary). There are only a few moments in the movies (haven't seen every TV episode) where one might think of such instincts. In LBT 2 we have Chomper bite Cera and shortly after he seems eerily interested in Petrie. When speaking of those two cases though it is crucial to keep in mind how young he was. Even with the extremely fast learning capabilities of the dinosaurs in LBT I think it is tricky to refer to the more conscious Chomper of LBT 5 and the TV-episodes on the basis of these actions on the first day.
But in LBT 5 we actually see him chase Littlefoot and the gang. We never know for sure what would have happened if it had been any other dinosaurs. On the one hand I am not sure that he was really out to kill. We know he can make do on smaller things and I have a theory on how Chomper acquired his knowledge of leafeater language that would involve his contact with at least Ruby before LBT 5, but possibly other leafeaters as well (which might in this case provide an alternative about the might have beens if Chomper had caught a random group of leafeaters).
Could Chomper bring down a Littlefoot sized dinosaur at all even if he wanted to? Perhaps so I think. I don't really know but if he really
meant it I do think Chomper's claws and teeth at the right spot could prove lethal.
Would Chomper eat dinosaurs (rather than insects) at all? This one I think is obvious. He mentions having had longneck for breakfast as an excuse for the longneck smell to his parents. His parents were not to the least surprised, so I guess we can take it for granted that he did eat other dinosaurs in his life outside the Great Valley and is deliberately on an unusual diet during the TV-series (even though he doesn't seem to have a problem with that). If we knew if Chomper's parents had brought down a longneck shortly before LBT 5 his comment would also enable us to draw conclusions on whether or not his parents would consider him capable of attacking and bringing down a longneck (obviously one not too much older than him).
LBT 5 also has this one scene in which Littlefoot invites Chomper to have a bite referring to the green stuff while by Chomper's look at the leg Littlefoot was gesturing with I can see where Cera's warning was comming from. Was Chomper misunderstanding the invitation? Did he think of taking a bit if so generously and selflessly invited? We will never know for sure. Given the fact though that he did try some green stuff during the song "Friends for Dinner" and hated it and given his still quite happy (almost watering mouthed) expression at Littlefoot's invitation I do not think a claim that Cera's warning was pure snide without any background could be upheld.
One question which I find even more acute than the question about Chomper's diet in the future is the question about his and his friend's attitude about it all. Of course we do have countless scenes in which the Gang makes half serious comments about it, but some of the main questions are still dodged. I have often wondered if Littlefoot ever thought about his mother in the presence of Chomper (ironically LBT 5 was the first ever sequel to mention Littlefoot's mother at all, but they didn't draw the line between that mention and the events of the story). Also one of the stories I was considering was about a situation in which the GOF would have to make a choice whether to help Chomper and his parents get of the island (where food was running low in LBT 5 already so it is safe to assume they would starve there before long unless they got away) thereby being responsible for the death of many more dinosaurs and other dinosaurs ending up in the same situation as Littlefoot did in the original movie or else being responsible for not helping a friend and his folks leaving them to their fate.
Having thought a lot about such a story I must admit that I do see why this was too hot an iron to touch for the movie makers. I admit that my story too would probably end kind of unsatisfactory with events beyond the control or decisions of the gang allowing for Chomper and his folks to get of the island.
