meaning of sentence

If I "try to wipe the grin off your face", I am trying, in conversation with you, or others present, to reduce in you what I perceive to be an inappropriate degree of self-satisfaction or glee or hilarity.

It is a phrase for use in conversation or very casual informal writing.

Unfortunately your story does not make sense and has some errors.

There should not be "so" before "certain".

Why did the boy's mother not write out a shopping list and give it to the boy?

If the items that the boy bought were "exactly what his mother would have bought" why was the meal unusual in any way?

The first paragraph talks about a hypothetical, fictional boy. In the second paragraph, all of a sudden, the "boy's mother" becomes the writer's mother, and the writer's brother is mentioned. (Is he "the boy"?). Why is he grinning?

One does not "feast" a lunch. One feasts (no quotes neded) on a lunch.

Thanks, Contrex.

You're very thorough so that my mistakes cannot escape detection by you. Many thanks for the explanation and corrections.

A boy was asked to go to the market to buy the necessary food items for his mother to cook lunch. However, he was not certain exactly what his mother wanted. So he bought what he thought would be a good lunch, including fish, prawns and vegetables.

That afternoon, a weird assortment of dishes was served on the table. No one spoke. His mother tried very hard to wipe the grin off the boy's face. Finally, everyone burst into laughter and began to take part in a feast they would never forget.