What are the visa options and requirements for expats changing from a non-O visa to a tourist visa in Thailand?

I've been volunteering with a Thai foundation and have a non-immigrant O visa through them. My husband and son are following me. We are getting ready to go to the US for a visit, but I'm ending my commitment at the foundation, which means our visa will be cancelled about 15 days prior to our departure. Since we won't be volunteering or working, just preparing for our visit to the US, a tourist visa makes the most sense to us. I was wondering the following:

1. Is it true that if we drive over a land border to renew, we can only get an extra 15 days but we can get 30 days if we go through an airport?

1. If we are changing visa types from a non-O to a tourist, do we need to go to a Thai embassy or would our visas simply be cancelled on our exit and then could we just get a 30 day stamp upon our return to Thailand?

1. Our daughter, who is 1.5 years old was born in Thailand and we were advised that she didn't need a visa. There is much conflicting information out there about this issue. Some people say that a child born in Thailand doesn't need a visa until they are 15, others say they don't need a visa until they leave the country. Can anyone provide clarity on this issue for me?

Thank you!