my mother in law is 80 or so (shes not sure) she speak with her friends and others exclusively in Khmer but can also speak, read and write Thai when she has to, My father in law was the same but he could read and speak and write official Khmer too, learnt in the temple apparently. My mrs and her siblings all speak Thai and khmer but none of them passed it on to their kids
So the in laws can only speak Thai to their grandkids, as they were never taught surin khmer by their parents, I think back in the day the Thai government discouraged people from speaking khmer now its all the rage apparently
Its interesting listening to people in the village talking to each other switching seamlessly from Thai to Khmer, a lot of local radio Djs do the same
I also note that my mrs is quite reluctant to speak khmer to her mother or anybody else if she is on the phone and likely to be overheard by none khmer speakers who she doesn;t know