Failed to get 2nd 6 month stay when reenter from Canada

My parents have the multiple entries visa to the US till next April. It is almost the end of their 6-month of stay with us, so they went to Canada in order to get a new I-94 for another 6-month of stay, but unfortunately the border officer refused to issue a new one but asked them to leave as normal.
Should I extend their stay now or have them go back to China and then return by April, 2006 before the visa expires. Which is better?
I appreciate very much if you can provide more info about the best option.
My email address: [e-mail]
Thanks!
Cindy