美國居留權

only for the children under age 18. under USA common law, children over 18 can be independent form their parant.
the fastest way to become the citizen is file the application after they had stayed in US over 4 years and 9 months. ( the proccess will takes 3 to 6 months ).
single and married is no different.

* above answer is for their children are in US only, if the children still live in HK. they need to follow the procdure to apply the immigrent first.

成年子女(21歲以上)不會即時都變了美國公民﹐爸爸和媽媽要去美國移民局申請他們的成年子女。 排期都要5年-8年以上子女才可有綠卡