Summer's coming to the Northern Hemisphere, and not-yet-outlawed, and-not-yet-safe-for-life-preserviing small-kids arm-surround inflatable "floats" or "floaties" have been recommended here, Cheap enough to be low risk for a trial and disappointment.
Something like these:
Note the plural: "These", so you a pair for $6. Where the chambers don''t communicate, you carefully cut between them to create a foot / .3M wide pillow.
Obviously you wrap them in a fabric cover. Maybe inject some down, despite the quality down shortage. Or glue feathers on. (Is this the DIY forum?)