I've been thinking about this for a long time, but I think I have come up with a solution that could make the Great Valley a realistic place to exist without upsetting the balance of nature.
As I think was mentioned earlier, there was the idea of disease raging the valley. Maybe most of the deaths were young dinosaurs, kids who were the most vulnerable. But then there is the topic of the bodies. Where do they go? Well if I remember correctly, some pterosaurs were scavengers, feasting on the bodies of the dead. If they were anything like vultures, they probably would strip a carcass clean no problem, which would help to eliminite the problem of where the bodies are.
But this alone may not explain the population control that is going on in Land Before Time. Now here's another idea to add on: Eggstealers. We see that it is easy for an eggstealer to take an egg, any egg it wanted, especially at night. Ozzy and Strut would have succeeded in each egg-theft attempt had it not been for Littlefoot and his pals. If they weren't involved, Ozzy would have stolen at least 3 eggs (one from a duckbill's nest, another from Ducky's nest, and a third from a pterosaur's nest).
The eggstealers also have an easier time getting into the valley, and they, being thieves, are expert at hiding and moving around without a sound. So I was thinking, what if there were a lot of eggstealers hiding away in the valley? They could all hunt at night, but with such a large number of herbivores, they would probably only be able to eat a small fraction of the eggs, perhaps just around 10%. But that 10%, combined with diseases, could be a way to allow the valley to remain the paradise it is but also be ecologically possible.