if they in fact grow up from one movie to the next, they do very slowly.
all of them constantly gain experience from every adventure.
I couldn't agree more Shinji-Lee. Everyone else had excellent points as well. I believe that they are definately maturing, as time goes on. A few things I've noticed point toward that notion very strongly. Here's three examples off the top of my head:
Ducky- In The Land Before Time VIII: The Big Freeze, Spike's, well, ways of doing things began to get on Ducky's nerves, which is typical for all siblings eventually. She ended up losing her temper and in the process made Spike feel unwanted. I'm stopping there because I'm proving a point, not summarizing a plot. Then, in the very next movie, The Land Before Time IX: Journey to Big Water, Spike wasn't being careful and knocks her off his back into the mud. Spike looks worried because he remembers from previously that those kind of things annoy his sister. But Ducky has matured because she doesn't get angry and even laughs to show that she really didn't mind.
Cera- In The Land Before Time, Cera has a serious problem admitting whenever she is afraid of something. I mean, she nearly gets a heart attack from seeing a sharptooth then tells the others how brave she was about it. However, in The Land Before Time V: The Mysterious Island, it shows that she has begun to look past her pride and admit that things do scare her, during the song "Always There."
Littlefoot- In The Land Before Time XI: Invasion of the Tinysauruses, he does what most kids do when caught doing something wrong: he lied to get out of trouble. Provided he does make things right in the end, but he caused a lot of trouble. But he's learning. In The Hidden Canyon, when Littlefoot and his friends (excluding Chomper) make a mistake by eating too many sweets and allowing sharpteeth in, lying doesn't even cross Littlefoot's mind. In fact, he decides to tell the grown ups what happened right away. If that isn't maturing, well, I'd really like to hear what is.
As you can see, although they are small steps, they are definately being made by the gang more and more as time passes. And whether they realize it or not, they are preparing themselves for a successful future in the process.