4 Weeks SE Asia w/ 12 and 8-year old

Hello!

I am planning a 4-week trip (Dec 2012-Jan 2013) to SE Asia with my 12 and 8 year old children.

Am thinking of flying into Bangkok and out of Phnom Penh. Now, trying to figure out how best to spend our time:

Bangkok
Khai Yai National Park
Chiang Saen
Phang Nga Bay
Chiang Mai
Elephant Nature Park
Vientiane
Vang Vieng
Pak Ou Caves
Hill tribe near China border?
Hanoi
Hoi An
Sapa
Saigon
Siem Reap/Angkor Wat
Angkor Wat
Phnom Penh

Am getting the impression this may be overly ambitious to cover Thailand, Laos, Vietnam, and Cambodia? We are not looking for a beach experience or a "vacation" per se, we want culturally rich and fun experiences and see and do interesting stuff. I know it will be more from a tourist-y angle as we will not be there long enough for full immersion. I know that 4 weeks is not much...but it's what we can do, so am looking to maximize that.

Any must-do's for first-timers/children? We have not traveled in this region before. Would appreciate experiences/advice. One of my children gets motion sick on long, hilly bus rides so I'm thinking that air/train is the best mode of transport.

Also, how much should I plan ahead? Some travelers say, "Book your first night at a hotel and take it from there." Is that really a good approach?

I would plan out at least the framework of your itinerary -- book major flights and hotels for at least the first night(s) in those places. Then you can work out other transportation when you get there.
As for the best itinerary, it might make sense to work with a local travel agent to book some things for you -- not only can they often get better prices on flights and hotels than you can do on your own, they will be able to give you an idea of what's too much and what is doable.
Also, not to add too much more to your itinerary, but I would also consider Ayutthaya and Sukhothai -- both of which are drivable from Bangkok and are Thailand's historical capitals.
Enjoy your trip!