A hot-air balloon flies because a flame heats the air inside the big balloon, making that air lighter than the cooler air outside so the balloon rises, and when the air cools the balloon slowly comes down.
One-sentence, kid-friendly explanation of how a hot-air balloon works: warm air inside the balloon makes it lighter and rise, and cooling makes it go down.
A hot-air balloon flies because a flame heats the air inside the big balloon, making that air lighter than the cooler air outside so the balloon rises, and when the air cools the balloon slowly comes down.