My husband and my son are Australian citizens. We are staying in Phuket and need to extend their visas since we didn't think about it before. Can they extend their visas for up to 30 days in Patong immigration or do they need to re- enter Thailand? 

Thanks in advance

You don't say what type of visa they have.  If Tourist or 30 day entry stamp then both can be extended by 30 days at Patong Immigration, cost 1,900 baht. You will need passport copies and original passport, fill in Immigration form while waiting. Might need a passport photo.

 

Patong opens 10.00, lunch 12.00 to 13.00. Closes early, maybe 14.00.

As LIK says, a little more information would be good, but if they are on a 60 day tourist visa, they can extend, and if they are on a 30 day visa exemption stamp. they can extend, but what they can't do is extend, an extension.