It's possible that they were unaware. I have doubts they heard the fight to begin with. Littlefoot's mother likely woke up, realized her son was missing, and went out to look for him. Littlefoot did in fact wander away quite far. In his own words, he said "why did I have to stray so far from home?" Or something along those lines.
The grandparents are seen in the earthquake yes, but that didn't mean they could do anything to help. Chances are, they were asleep while their daughter left, and when they woke up to find her, it was too late and the earthquake happened. They couldn't make it over in time and thus were separated. There was no way they could cross that canyon.
I have doubts that they were distracted by anything. I think the most logical conclusion is that Littlefoot went too far, his mom followed, not waking her parents, her parents wake up and find that she is gone, by the time they get anywhere near where their missing family members had one, the earthquake started and split the land, separating them.
