Thailand and Cambodia. We went in 2016 when our kids were ages 11,10 and 7.
AIR
If you are flying from the United States like we were, this is a very long trip to get across the continent. Make sure to find the fastest route there. It may cost you a little extra, but worth it in the end. Luckily because it was international, in-flight entertainment meant we didn't need to worry about ipads or iphones or downloading millions of shows or movies. The kids had access to so many movies and tv shows. This was also a problem, because it meant they would be stuck and probably not get any sleep. Set limits. Make sure they get 5-6 hours of no TV time and hope that they get some sleep during this time. Or else you are doomed. As far as food is concerned, it is always a good idea to pack some snacks. We flew Air Emirates, they kept us well nourished and when it wasn't meal time they had baskets of fruit and candy. They also had drinks available the entire time. Of course, our kids hoarded the candy.
