Audio problem: For reasons unknown, the child's voice throughout this episode is played by an adult actress trying to sound childlike - and the unconvincing imitation is painfully obvious. (00:19:25)
Continuity mistake: During Rod Serling's opening narration you can see over his shoulder part of the outline of the portal to the 4th dimension, which would be drawn there later in the story.
Answer: It's explained in the episode. The aliens are experimenting to see how humans react to various scenarios. This just happens to involve them using super strength.
Thanks. It's been a long time since I saw the episode so I forgot.