Parents visiting

How long my parents will have to wait for re-entry, once they leave US after their six months stay? Both of them have recently obtained 10 years visitors visa. Since all children live in US, there is no one to look after them in India. Can they visit neibhouring country (Canada, or Mexico) and re-enter with new I-94 with an extended period? Is applying for extension of stay isa more appropriate approach? Mother left US after staying with us four six months about 5 years ago.
 
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