.......I think any five year old knows the sun is the source of day light.
I am highly dubious of your claim that any 5 yr. old understands that the sun is the source of light. If you were to ask a child of that age "Why is it daylight?" they would probably not understand the question. It is daylight outside because it is light. In fact it is obvious from the bible that the author did not understand the relationship that daylight had with the sun. If they did they would not have stated that God created "light" THEN later he created the sun.