Theory:

The poet wants to emphasize on not falling prey to unnecessary flattering. Mainly, she wants the children to understand never to fall into the traps of the wrong people. The poet gave an example of the spider and the fly to make the children recognize that these wrong people will try to woo you by talking sweetly about you, but they should keep their eye, ears, and heart closed. It is because we see the wrong people who give chocolates or candies to small children to kidnap them or tell them that his/her parents have sent them to pick them up from school. So, don't trust these people blindly. Think, take the help of some adults known to you.
 
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The story is about a spider and a fly. The spider wove its web and asked the fly to enter its parlour, as it has some goodies and that it has a mirror to show the beauty of the fly. Though the fly had simple wings and eyes, the spider flatters the fly by saying that the wings of the fly are shining like silver, eyes are like diamonds, and its antennas are looking like a crown.
 
The fly falls into the trap of the wily spider and gets killed. So beware of such people and do not fall for wrong flattering like the fly.