A 5wt, rubber legged poppers and a pond full of bluegill worked for my kids. Lessons were given on the lawn first, just to get the fundamentals together before the trip (for safety's sake). My son eventually stuck with it and got his share of nice stripers on the fly.
The most important thing to teach them first is safety:
a) cast with the wind blowing away from the casting side
b) turn around, reach across or cast lefty when the wind is coming the other way
c) wear eye safety when learning
d) pinch all barbs
e) avoid swift currents, slippery rocks and angry surf
f) did I mention casting only as the wind allows?
Good luck with it!
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum