Is there a solid way or rule which can be used to choose between possessive and adjectival noun?

Question

I have this senetence marked as a mistake "might waste their children time", and it should be "children's time". I looked for a grammar rule, but I was not able to find it. Is there a solid way or rule which can be used to choose between possessive and adjectival noun? for example:

• For School bag or College student: adjectival nouns are used, and I cannot imagine that possessive is possible here.

• People’s behavior or people behaviour : I think both are correct and they are the same thing.

• Children time is wrong: But what about "dinner time"? If the dinner can be modifier for the time, why children cannot?

Help!

Answer 1

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.

Answer 2

Yu are correct that the form should be "children's time", never "children time". You are also correct that the time does not "belong" to the children any more than the student belongs to the college. I don't think there is a single rule which indicates when a possessive form should be used. However, when the first element is a person or group of people, the possessive form is more likely. But that will not always lead to the correct form.

One might say "the college's student" when referring to a particular student of a particular college, but that is a rather different usage than "college student" which simply means anyone who is attending college.