Try this suggestion. Go to a dive shop and purchase a childs sized inflatable snorkel vest. It will pack in the space of a tee shirt and will be more than adequate to assist
your six year old in the water. You just inflate it to the point where she is comfortable. This is not a toy , this is serious swim assistance. Child size snorkel vest will probably run about $35 . This is not a Wal Mart type item, find a real dive shop (or purchase on line).