first off, make sure each of them are on experienced, quiet poines, make sure they wear helmets, and teach them SAFETY! (someone else said this). i would suggest throwing a few games in there... that require skill at the same time. (for example, simon says. i think someone said something about balance such as touchung their toes or the horses ears. twist that into the games.) also, if you have any other place to ride other than an arena, take them out there (trails, property, etc.) but, still practice steering and balance.
i like the lunge line idea, too. i learned to trot by practicing correct posting at the walk. i would also say you should work with one girl and your friend to work with the other girl, (whichever one of you is more experienced english, work with the younger girl. whichever is more western works with the older girl.)
anyway, Good Luck!
He carried me away from all my fears, and his mane was there to wipe my tears. -RIP Vegas, my best friend.