Why do Ducky and Petrie's speech patterns sound like borderline baby-talk while Littlefoot, Cera and Spike (presumably) talk normally? Because they are of different types of dinosaur. Ducky, unlike the other three is a bipedal dinosaur, so her dialogue is more childish but still pretty understandable. Petrie is further removed being a 'flyer', primarily a flying dinosaur in a group of land-dwellers, so his dialogue would be more strange and out-there. For those of you who say this makes no sense in terms of realistic language barriers, keep in mind that the movie is already farfetched for having all of these species alive at once anyway, and that this addresses the issue of different dino-languages while still being able to be enjoyed by younger viewers. As for there parents and other dinosaurs, all of those cases are in the sequels and may not be what Bluth intended. The only adults to speak in the first movie were Littlefoot's mother, Cera's father and Rooter, all fellow quadrupeds.
Pterano from the 7th movie. His mood flip-flops all over the place from scene to scene, he's prone to little outbursts of anger (namely towards his lackeys when they mess up/push his buttons), is mostly incredibly averse to violence (save for little dope slaps on Rinkus and Sierra, again), never mentions or acknowledges the massacre he led his followers into, and appears to have a full-fledged flashback when Ducky is in danger of falling off the mountain. Any of these symptoms sound familiar? They should. Pterano potentially has PTSD.
The Pachycephalosaurs. The alleged portrayal of these dinosaurs as "carnivores" in the film is often regarded as a research flub by armchair paleontologists with sticks up their asses, but it's actually a case of Fridge Brilliance and Fridge Horror. There is a deleted scene from the film (which was included in the novelization) where Littlefoot and his friends meet a herd of Pachecyphalosaurs (identified as "Crown Heads") who refuse to share their food with them because they're not of their kind. This establishes the Crown Heads as herbivores and as racists. This is where the Fridge Horror comes in; the Crown Heads that attack Cera were most likely not motivated by hunger, but by racial hatred. They were terrorizing her For the Evulz, as seen when they loom over her menacingly when they have her surrounded just to savour her fear, instead of immediately tearing her to peices as a pack of starving predators would do in this situation.
They may have even been the same Crown Heads, since Cera insulted them in the aforementioned deleted scene, and may have gone after her for payback.
Why is ducky named "ducky"? The movie never shows birds existing at the same time as the dinosaurs. Although in retrospect it becomes accidental Fridge Brilliance as the earliest waterfowl already existed when Ducky's species was alive.