I always got the feeling that they'd slept through the fight, while Littlefoot's mother roused early for some reason, noticed Littlefoot was gone, heard the screams, and charged off to save him. I suppose it could be noted that Cera's parents didn't come to help, so if they slept through her yells than it's not too hard to imagine Littlefoot's grandparents doing the same.
It's worth noting that Littlefoot, his mother and Cera all end up quite far away from their original nesting site as a result of the battle (besides, isn't that how they end up on either side of the divide; the nests were on one side, and Sharptooth chased them so they ended up on the other side?), and I seem to recall there being a shot of Grandpa and Grandma Longneck looking around as the Earthshake begins? Chances are that they were woken up just before the Earthshake began, and as such they didn't have any time to go after Littlefoot and his mother.
Incidentally, this would have meant that they wouldn't have known what happened to Littlefoot or his mother, but they would have been able to see her corpse from that height, and this might have been why they moved on with the others; they could have assumed that Littlefoot died with his mother, or that she died while trying to save him.