Stopover in a Quiet Town - S5-E30
Revealing mistake: The actors are supposed to be in a play set for the huge child, but when she reaches down and picks them up it is obvious that it was filmed in an outdoor setting in a town much bigger than what is supposed to be the play set.
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.