Is it possible that you are worried about losing your daughter as she grows older? At 12 I expect she's right on the edge of puberty if she hasn't started already, and there seem to be a lot of parallels in your dream:

As she gets older she is wanting more and more to "steer" her own life, and even though you're still mostly in charge, you still have to give her that control sometimes, even if she might get hurt in the process (driving into the cliff)

She is taking more risks (the rocks) and wants your approval as she does it. To me, the cliff suggests teenagerhood - she didn't scream or crash, she was just gone from you, the way a teenager abruptly starts to shut their parents out and assert their own identity (don't worry, I hear they come back eventually!) Similarly, her coming in but you and your mother knowing she isn't really there is like how your teenage daughter can be standing right in front of you, but she just isn't the little girl you used to know. Something that a mother and grandmother may feel more keenly than the men in the family, so you feel the urge to hold her close and cherish her while she's still your baby girl.

Of course, I could be completely wrong. Either way, it is a very sad dream.