They weren’t asleep at the moment the Great Earthshake hit? Why did she bother to go fight Sharptooth alone? Also, if she had been looking for Littlefoot and had then heard his cry for help,it’s also a mystery why she didn’t get her parents to go help her look for Littlefoot.
Granted, had they gone, they might have saved Littlefoot’s mother, but that would have the following possible side effects:
1.) Given how Grandpa Longneck was bested by Chomper’s mother, I think Sharptooth would have gotten the upper hand on him and killed him. Sharptooth may also have killed Grandma Longneck too.
2.) Had Grandpa Longneck died, that would have meant that Cera and her father died in that fire in the third movie and Tricia would have never been born.
3.) If 2 happens, then Cera wouldn’t have saved Petrie in the 7th movie and he would have died from that rock. If that happened, Pterano would have stayed banished for life, if not been executed for messing up things and bringing about Petrie’s death.
4.) Since Littlefoot would not have gone to the Land of Mists with Ali, he and Ali might not be as close, which might mess up a potential future the two could have had together.
However, I don’t see how Bluth, Lucas, and Speilberg could have foreseen these issues when making the original Land Before Time, so it still doesn’t explain that plothole.