the behavior of people
people's behavior. It's the behavior that people have. Because we say it is behavior that people have, have is considered possession.
people's is the possessive there.
to waste the time of their children:
to waste their children's time. It's the time their children have.
If you said, children time, that would be like adult time. Families have adult time and children time. [Those are a possibility but not usual. children and adult would be adjectives in this case.]
If you say: people [human] behavior versus animal behavior, there people and animal are used as adjectives. Though people behavior should really be human behavior.
[trick: Where the Spanish or Portuguese uses de, you can bet the English often is possessive though not always, alas.]
a college student is not a high-school student, but a college's students are the ones who go to some college.
They are students of some college. [can't resist: os estudantes universitários versus os estudantes da universidade. That would be like Spanish, too]
- dinnertime is one word, not two.
It can get tricky in English: we do say the car door though door of the car is not a mistake. Just like: bedroom door. Parts of houses are used as adjectives. The attic room, the basement room, etc.
I prefer to say that in college student, a noun is used as an adjective or adjectivized noun.