Here are some tips to get Cubs & Scouts interested in knots.
Know the knots and lashings well yourself, firstly. If you don't know the knots how can't you teach the members properly.
Have decent rope and string to start with. Its no good teaching knots with rope that is tatty and scraggy to start off with, or its half a mile long.
Give reasons for doing each knot, explain its purpose. Its no good doing something if you don't why your doing it.
Then do something fun with the knots, make a game out of it, or building something that is exciting like catapaults.