Inflatable toys, chairs and beds, kids castles, pools, stuff like that.
I understand and seen people use ordinary polypropylene? trashbag sheets or tablecloths for inflatables where the pieces can be joined by putting a paper between a hot iron and the sheets.
But from my experience that material is not thick enough or durable enough, it's very easy to get holes on it. Not that commercial kiddie pools or chairs are bulletproof but there's no denying they are more durable.
One chinese supplier said "PVC Tarpaulin" or "600D Polyester Oxford Fabric" is used for their large inflatable pools but I don't know if it's too much, too fragile and cheap or there are easier sourceable or better alternatives at the same price range. By the way tarpaulin pieces can be welded with ordinary heatgun and a roller.
Wikipedia page on inflatable castles mentions "PVC or vinyl and nylon" but those are loose terms describing many kinds of materials containing one of the above materials. For example PVC can be used to describe a rigid pvc plastic sheet, flexible pvc foam or pvc tarpaulin.