A former Cubmaster here (and current Trainer) --

One of our cub camp directors had great success teaching knots by first teaching the names of the part of the line -- running end, standing part; and teaching the basic parts of a knot and used memonics i.e- a bight (looks like a mouth that bites...:D), overhand, and underhand loops.

Using the line names and building blocks really helped with the reef (square) knot --
Most people teach it as "left over right, right over left," but most people's muscle memory causes them to tie both halves the same way.
Our camp director used:
- start like you are tying your shoes,
- now take the rope in your left (or right) hand and make a bight against the standing part (point and demonstrate).
- next tuck the end in your right (or left) hand under the bight.
- pull tight and you have a reef or square knot.

He had them tying the knot behind their back in about 10 minutes... definite confidence builder for the boys.