We spent a month in Cambodia last year and three weeks in Vietnam. Both were really easy countries to travel in with kids and I think you will have a wonderful time.
You haven't mentioned how many weeks your trip will be? Are you planning on flying some of those distances or catching the train/bus?
We actually preferred Cambodia to Vietnam. It was a lot easier in Cambodia to find people who spoke English well, pharmacies were easier to find, the kids liked the local food better, travel distances were smaller and towns/cities was quieter. It probably didn't help that we were silly enough to visit Vietnam at the height of their summer when it was ridiculously hot and humid. Sightseeing when it's 42 degrees and 90% humidity is not a lot of fun! But just logistics wise we found Cambodia easier and more fun with the kids.
If you are going overland from Ho Chi Minh, perhaps look at a southern route around Tonle Sap to Siem Reap rather than through Kampong Cham. We really enjoyed Kampot/Kep for the quiet villages, caves to explore and lazy days on the river, Battambang for the bamboo train and learning how rice paper/stickyrice/silk is made and of course Siem Reap. It's a longer route than the one you are planning so it may not work but we really enjoyed these places.
Good luck deciding!
