You could make something quilted to float by putting something in it to make it buoyant like foam or air bubble but it wouldn't be waterproof. If they are concerned about their phone or tablet sinking to the bottom of the lake then this might do the trick but the phone or tablet wouldn't work after that because it had gotten wet. You can't make a waterproof quilted container. Electronic devices are very sensitive to water or liquids. I spilled no more than 3 drops of water on my keyboard of my laptop from my wet hands as I walked by and now the shift key doesn't work for 2 letters. Thank goodness the caps lock key still works so I can capitalize.
My cousin was showing her new iPhone to one of her adult kids. They were sitting at the table having lunch and as she passed it over to them it slipped out of her hand and fell in a glass of milk. Good bye new iPhone!
Tell your kids to be careful with their phones or tablets and for THEM to be prepared to buy a new one if they are careless.