The requirements to claim a dependent are not as simple as being in the country for a specific amout of time. Worldwide income and tax issues are considered.
A visitor should not need an ITIN number and these are issued when taxes are filed.
On a philosophical note, when they obtained a visitors visa, they stated that their ties to another country are greater than to the US. If you claim them as dependents, does not that prove that they misrepresented the statement since you are claiming them as your legal dependents who would be seen to have closer ties to the country in which their financial supporter is living?