I bought this for my 4 year old daughter to help her to learn to swim. I have tried the arm bands and even the floatation vests for kids. The arm bands impaired the movement and the flotation vests were unstable especially for children and dangerous in my opinon, because if they put their heads in the water the vest hindered them from lifting their heads out of the water.
However the swimfin is abosulutely amazing. My daughter normally nervous when she is in slighltly deeper water, let go of my hands withing minutes of wearing the fin.
After that I just left her to it really, obviously in close proximity. She was swimming up and down the pool, even at the deep end confidently (albeit in a more vertical doggy paddle style)
Now I am teaching her to move her arms out of the water and there is no restriction in movement.
I would this recommend this for any child learning to swim. (it goes without saying for the parents, that supervision is still needed!)