From my answer to this question: Why no 的 in this sentence?
The possessive '的' after a subject is usually kept, if it is a 'true possessive'(something you actually own)
The possessive 的 in 我的車 cannot be omitted and make it "我車"
On the other hand, if the '的' is a 'relative possessive'(something you don't actually own) can be omitted. For example: you can omit the '的' in '我的母亲' and write 我母亲.
The possessive 的 in 这是我的太太; 他是我的女儿; 你的国家; 你的公司 can be omitted. You can write 这是我太太; 他是我女儿; 你国家; 你公司 (if you actually own the country or the company, then the possessive 的 should be kept)
我口 to mean "my mouth" is wrong apparently
Because you own your mouth, therefore 的 is a true possessive and cannot be omitted
"中国的女孩" means 'China's girls' (the girls belong to China, therefore, 的 here is a true possessive and has to be kept)
"中国女孩" means 'Chinese girl' (中国 here is an adjective 'Chinese')
If you omit '的' in "中国的女孩", the meaning would change
我儿子 owns the 巧克力, therefore, you cannot omit 的 and write 我儿子巧克力
More Examples from my answer to the linked post:
我(的)妈妈的房子很高。 can be reduced to: 我妈妈的房子很高。
You don't own your mother, but your mother own the house
我(的)牛(的)角很长。 cannot be reduced to: 我牛的角很长。 or 我的牛角很长
you own the cow and the cow own the horns