Well, if you are focussing on theme, you will want to consider how this story uses locations symbolically to highlight the theme Lessing is trying to convey. You need to understand that this is a short story that is taught to show symbolism in use, and clearly the symbolism is linked to the meaning and message the author is communicating. I will point you in the right direction to begin considering these important questions.
You need to consider the three settings firstly: the wild beach, the safe beach, and the tunnel. Tell what you think each setting must symbolise for Jerry. Then consider the larger symbolic meaning of Jerry's passage through the tunnel. What does his trip represent? Why is it such a powerful symbol? How is the Jerry after this trip different from the Jerry before?
These should help you get started, but just to give you an extra helping hand, consider how the wild beach is described:
He went out fast over the gleaming sand, over a middle region where rocks lay like discoloured monsters under the surface...
Compare this with the safe beach where his mother is:
There she was, a speck of yellow under an umbrella that looked like a slice of orange peel.
You would do well to consider the dangerous, scary but adventurous way in which the wild beach is described and the domesticated, safe and boring imagery used to describe the safe beach. I mean, which would you rather have - rocks like monsters or orange peel?
Pick out quotes to consider the symbolism of the tunnel and hopefully you will find lots of examples to show and prove that this story is all about a coming of age - and of course, the passage through the tunnel symbolises that transition from child to adult.