for doing an instance...
I would guess that the targets need to be valid to both the lowbie and the 80.
if it's too low to the high charcter is not good, if it's too high for the low character it's not good.
the min valid level for the 80 character is 72.
So, if 72 isn't too much higher than your 68, it should give some pretty good points.
Drak'tharon or maybe the old kingdom instances might work.

You could probably put the two lowbies in a team together, and follow them with the high characters and kill mobs over 72 in dragonblight for good rested XP.