Can Parents第三国签证after 6 months

Please help:
Visiting parents can have the multiple entries for one year, but they can only get 6 month - stay when they enter US, does anyone know if they can go to Mexico or Canada at the end of valid six months, and re-enter with the additional 6 months?
I want my parents can stay with me as long as one year, but do not want to try extending at the end of first 6 months since it is very risky.
Is re-entering from 第三国 with the valid multiple entries possible, or better than visa extention?

Thnaks!