The sixth Land Before Time movie is "The Secret of Saurus Rock", and this is the last movie in the series which I saw, while growing up as a kid. All I remember is that I saw it once, and I didn't like it because it focused too much on the Lone Dinosaur. At the time, I didn't understand why they put an old west hero into a dinosaur movie. I still don't understand it, in fact.
The story goes that a huge mountain named Saurus Rock appeared in the valley, after the Lone Dinosaur saved everyone from a sharp tooth. If the monument is ever destroyed. A cowboy dinosaur called Doc appears shortly after this, and Littlefoot instantly hero-worships him. Not sure why. Doc hardly ever talks, and pretty much whatever he says has the undertone of, "Kid, leave me alone. I hate talking to people." I'm told that he acts just like John Wayne, but since I've never seen John Wayne movies, I can't confirm this.
Cera has twin nieces, named Dina and Dana. Who are the twins' parents? That's unexplained. Just accept the fact that Cera is watching two toddlers, and move on. She yells at them in a fit of anger, and they run away to Saurus Rock. Our heroes give chase, and they accidentally break off a tooth-shaped part of the mountain.
Bad luck soon curses the entire valley, and an angry mob decides to run Cowboy Dinosaur out of town. Littlefoot has an emotional meltdown, and he decides to fix Saurus Rock by putting a dinosaur tooth there. Which requires getting a tooth, from a giant dinosaur that could eat him in one bite. It's not a smart plan, especially since the allosaurus in question is alive. Littlefoot's grandfather and Doc work together, in order to save him.
Littlefoot realizes that he doesn't need an imaginary hero to worship. His own grandfather is pretty darn cool. And that was an okay moral, I suppose.
I'm not sure how to rate this movie. It was fine, but nothing really stuck out as particularly good or bad. I'd say it's average, and I probably would have liked it better if the new character focus was someone besides John Wayne parody. That led to Littlefoot singing an old west song, which felt out of place. As for Cera's unexplained nieces, their main purpose was to be cute and advance the plot. They did that well enough. They don't talk (besides baby talk babble), so they don't really have personalities or memorable characteristics. I wonder if we'll see them again.