Earlier quoted context omitted.
Personal opinion, based off my own experience and teaching other children how to swim: Flotation devices have no part to play in teaching how to swim. Parents (or teachers) holding their children and teaching them how to float is step 1. Only after the child can handle themselves in water (float, know when to breathe) should they be playing with flotation devices.
We should clarify that PFDs (life-jackets) are probably good for young children and new swimmers. It's the floating toys that might actually be harmful. And one skill that swimmers need, but cannot really get with an float-assist device, is putting their face in the water. I watched an adult friend learn to swim, and this was REALLY hard for him. Crazy enough - he was ex-Royal Navy submariner, so he had passed basic…
We used kickboards for this exact purpose. So you can float face down with arms outstretched holding the board. You can transition to freestyle swimming taking one hand off the board at a time.