Passively floating on your back is hard for skinny folk! Much easier to backfloat while moving, and it’s still a much lower energy activity than treading water upright. I wouldn’t recommend a kid trying to practice backfloating while holding still unless they’re naturally buoyant. Instead, the question is ‘how little energy can you expend while staying up’, and ‘tracking your position by looking at the ceiling so you don’t bump your head while doing backstroke laps’. The faster you go, the easier it is to stay up, but the more energy you expend. There’s a comfortable medium that’ll be different for each person, and change as their body changes.
Passively floating on your back is hard for skinny folk! Much easier to backfloat while moving, and it’s still a much lower energy activity than treading water upright. I wouldn’t recommend a kid trying to practice backfloating while holding still unless they’re naturally buoyant. Instead, the question is ‘how little energy can you expend while staying up’, and ‘tracking your position by looking at the ceiling so you don’t bump your head while doing backstroke laps’. The faster you go, the easier it is to stay up, but the more energy you expend. There’s a comfortable medium that’ll be different for each person, and change as their body changes.