What is the significant difference of events in the lives of the two young characters of "The Flower" by Alice Walker and "A&P" by John...

The main difference between what occurs in the two young lives of the protagonists of Walker's and Updike's stories is that the loss of a romantic notion of life is forced upon Myop while Sammy brings his crash into reality upon himself.

That Myop dwells in a childish world of her own is evinced in the opening lines of "The Flowers":

It seemed to Myop as she skipped lightly from hen house to pigpen to smoke house that the days had never been as beautiful as there.

For, even though her parents and she are obviously poor sharecroppers and their lowly cabin has "rusty boards," Myop is not affected by such humble surroundings because these are all she knows in her innocence. With pigs and chickens and crops, the family probably has enough to eat, as well. Besides, she has the beauty of nature around her in which she delights. So, it is only when her innocent notions of life collide with the reality of man's inhumanity to man that the romantic cloud under which she dwells is harshly removed from Myop.

Similarly, Sammy's view of life is clouded by his youth in which he, too, has not yet been confronted by harsh realities. While he is critical of the middle-aged women--"houseslaves"--with their curlers and "varicose veins mapping their legs," Sammy is completely smitten when the girl he names Queenie enters the grocery store in her swim wear. He describes her in desirous tones, and concludes that she is "more than pretty." Then, when the store manager Mr. Lengel approaches Queenie and her two friends, telling them that they must leave because policy does not allow anyone to enter the store with their shoulders uncovered, Sammy acts according to his romanticized perceptions. He decides that he must rebelliously be their heroic defender.

That's policy for you. Policy is what the kingpins want. What the others want is juvenile delinquency.

Believing that he will impress the girls, particularly Queenie,  Sammy decides upon the chivalrous gesture of quitting his job in order to put on display for the girls his disapproval of the rules.

However, Sammy's romantic notions are destroyed once he is outside the store. For, his act goes unnoticed by the girls who are "gone, of course," as Sammy notes. Alone outside, Sammy looks back into the store and suddenly realizes the rashness of his romantic and voluntary act:

...my stomach kind of fell as I felt how hard the world was going to be for me hereafter. 

Certainly, an examination of the two stories reveals that the cultural differences of the two characters is apparent in the limitations of life. For those like Myop's people, poverty and cruelty are often imposed upon them, whereas for people like Sammy and the three girls there is more freedom to act.