Showed BTTF and BTTF2 to my 9 year old twins a few years ago. They loved the films. Not sure what they didn't get - even the concept of alternate timelines seemed fairly well absorbed.

The main things missed are probably all the various in-jokes, but that's not really important to the story.

Also "we'll travel to the FAR FUTURE" being when they were two years old has to be amusing (and sobering, to us olds).

Even a lot of people who lived in that era don't get the Pepsi Free joke since it was a only know by that name for part of the 80s and overshadowed by the larger themes of the Cola Wars.