Just hand both UK and Thai passports in when you leave going through immigration. A Thai cant overstay in Thailand and they wont be interested if she is dual citizen.
Is her Thai mother travelling out with you? If not make sure you have your kids birth certificate with you and a letter from her mother allowing her to leave the Kingdom.
If mother is with you then only birth certificate is required.
The first time we took our son out of Thailand my wife was questioned by the IO and asked if she was leaving on her own will or was I forcing her to leave.
There must be some weirdos out there forcing their Thai wives and kids out of the Kingdom if she was being asked this.