T-rex actually could swim
They found out sometime around 2007 that Tyrannosaurus rex could actually swim so Sharp tooth should have been waiting for them at the bottom of that hill when they thought everything was okay.
Some of you might say it was the rock that killed him that doesn't seem likely since he fell down a large hole and was "knocked out" for some period of time so why should some rock being pushed on him be any better?
Either way, Littlefoot's logic was "He can't swim with those scrawny arms."
Scrawny arms or not, Sharptooth could swim and he should have been able to and they would have walked down from that hill and turn the corner and boom sharptooth is right there, making them lunch.
Only people with money say that money doesn't matter.