One way to think about your question is not in terms of "morals" or "themes" but "how might the author want the reader to respond to the story?" or "what are some ways readers might react to this tale?"

Some possible ways of reading the text--and I'll try to phrase them as aphorisms--are that life moves quick; it is only after much of our lives have been lived that we understand how time has passed and how much has changed. There is also an aspect of the old saying "everything I learned about life, I learned in kindergarten": in the narrator's "half day" at school, he experiences the rules, structures, and social contexts that will last throughout his life. Notice how, in that morning, he does all the things he will repeat as an adult: he works, he plays, he loves, he follows rules, he breaks them. His school days create the narrator.

Also, leep in mind that Mafouz wrote the story in the 1940s, and Egypt was just beginning to develop many of the modern conveniences--cars, electronics, etc.--common to America by that point. For many countries in the 20th century, the change from a simple rural society to a complex urban society took place very quickly; many Egyptians, including Mafouz, likely felt confused about who they were and where they belonged. Mahfouz may be bemoaning such an occurrence, as it may diminish some of the beauty or significance from an individual's life.