Park's bench or park bench

In the following sentence, why hasn't an apostrophe been used in the phrases "park bench" and " children play" ?

* We sat down on the park bench and watched the children play.

Answer:

Easy answer first: in "children play", play is a verb, unless you mean "play = a stage production with actors." e.g. "the plays of William Shakespeare.", in which case it does need a genitive 's'.

So, let us look at "park bench"

In English it is very common to use two nouns together where the first noun
(i) is uncountable and
(ii) acts like an adjective:

The language department is over there. = The department associated with language .is over there.
.....<noun1> <noun2> is over there... = The <noun2>.... associated with <noun1> is over there.

We sat down on the ...park ......bench
We sat down on the <noun1> <noun2> = The <noun2> associated with <noun1> is over there.

This construction is used where there is not any real ownership. The first noun is used to describe an attribute of the second noun. The first noun doesn't say who/what owns the second noun.

We sat down on the park bench
We sat down on the wooden bench
We sat down on the newly made, iron bench

The bold phrases are all adjectival.