It's the same as if the ball goes in the stand and someone holds onto it. You throw another ball on. If that doesn't happen then Chelsea have a legitimate claim, and the ref should add the time on. It then becomes the fault of Swansea if a match ball is not provided to continue.
The kid is obviously some kind of gremlin, but the ref just adds another 20 seconds on and it's no issue.
I do feel sympathy with Hazard for kicking him, though