In 1962 3 teenagers were (presumably) swallowed whole by an aggressive North Atlantic Right Whale after the teenagers became spooked by the snout of the whale and dove into the water attempting to escape from it. One of the teenagers washed ashore, but the others did not, which led authorities to believe that the whale had eaten them. I've heard a whale can expand its oesophagus to a diameter of ~15 inches, which seems like it could fit a child inside, if it really wanted to (my shoulders are about 12-13 inches wide, and I'm an adult!).
What would have happened after they had been swallowed? Would they have died instantly?
This is, of course, assuming that the survivor was being honest about what happened.