To swim backstroke properly you have to have a strong leg kick.
There is no benefit to introducing the arm action before you have established a sound kicking action and a flat body.
Legs only on the back “Chin up, Tummy up, toes up”.
Keep doing kicking drills until they can swim unaided doing legs only. Use woggles then floats.
If they can all swim with one float you can use play Free willy
When they are ready, move on to Full Stroke backstroke.
Have the kids standing still at the wall straight “like a soldier” , hold one float on the chest and copying teacher
“ear, side don’t forget to kick”
Now they chant and you demonstrate:
“ear, side don’t forget to kick” ,
“Can’t hear you..?”
Give support to the head and guide their arms where needed.
Repeat using one arm at a time, “Stiff, straight arms, like a soldier”
After several repetitions, you can demonstrate and let them try full stroke, but still pausing after each arm to say the mantra:
“Don’t forget to kick”
Only use the same arm on each width, keep it simple. Vary the mantra with any relevant teaching point, like
” Ear, side, can’t see your toes?” “Ear, side, where’s your belly gone?” “Ear, side, reach for the sky“
Only introduce advance stuff like little finger first/continuous arms in the higher levels, breaststroke or butterfly.