I have a rough outline of a 4 month family trip in SE Asia. 2A one 10 year old. Mid Dec to mid April.
Generally I have: Vientiane then up to northern Laos, slow boat across to Thailand. A few weeks in northern Thailand then overnight train to Bangkok. Across to Siem Reap (am quite unsure of how to organise more time in Cambodia) and then into south Vietnam (maybe Phu Quoc?) then a slow journey north, mostly via train hops and then exit from Hanoi.

Key experiences currently include trekking northern Laos, the Mekong slow boat, overnight train, Angkor, a community based tourism home stay in Vietnam.

I've hit the point where I've done so much reading that I am 2nd guessing myself. I am hoping some more experienced long trip travellers might have some insights for me.

We are looking at a budget of $250 AUD per day (which at the moment is a depressing $150 USD) to cover everything so hoping to find low cost accommodation, but with chunks of time where we can have 2 bedroom places to get some space from each other.

Questions:
How long per stop do people suggest? Obviously this will vary and there might be spots where we stay just one night because we are on our way to somewhere else. Should we aim for an extended stop somewhere mid trip? Or just continue with 5-8 nights all the way through? Or longer stops everywhere?

How to sort out transport from Bangkok to Siem Reap - I'm wanting to avoid super long transport days that involved buses....

Cambodia recommendations past Siem Reap - Cambodia was a late addition to the itinerary and now I'm just lost.

to Phu Quoc of not? I think the kid would love the aquarium and amusement park, seems like it might be a bit expensive accommodation wise, maybe so a quick trip in and out?

What key experiences with kids have people loved in Vietnam?

Thanks so much for reading my essay!

You can fly from Bangkok to Siem Reap; flights are about an hour, the airport in SR is about an hour from town.

How much time to spend in Siem Reap depends on your interest in Angkor sites. Angkor Wat and the temples / sites nearest Siem Reap can be seen in 2 or 3 days -- be sure to see the museum there, too. I was interested in seeing a much wider range of Angkor ruins and so spent 9 nights in SR, visiting a number of distant (and magnificent) ruins.

Phnom Penh is an obvious choice for another place to visit in Cambodia.

I didn't write a trip report, but some of my notes / observations might prove useful:
Finally, Cambodia!

Southeast Asia is backpacker heaven, and most people never spend more than 3 days in one location. I know when I go, the longest I have ever spent in one town/city is 4 days and that was only because that included my arrival and departure days in the arrival city + 2 days exploring that city. 2 days is more than enough for most all destination that are not a major city if you really want to see as much as possible, but I would do 3 days in each location that way you aren't losing so much time traveling. Make sure the destinations you are going to are visited in order based on distance so that you your vacation time lost due to transpiration is kept at the lowest possible amount. Make sure you are taking that into account, as some destinations are an all day thing to get to, and others are a 45 min flight and you don't lose but a few hours of time from check out + flight + check in + adventure continues.

My answer is 2 days min, 3 days is perfect, 4 days is too much unless it's a massive city that you have an "interest" in fully exploring.

Here is an example of a 30-day Southeast Asia Travel Itinerary that I am going to do and traveling like this is very popular in the region. 3 months, there is no reason you don't see at least 30 destinations.

Your itinerary looks great! For Bangkok to Siem Reap, take the train to Aranyaprathet, then taxi to the border. Beyond Siem Reap, consider Battambang or Kampot. Phu Quoc is fun but pricey—2-3 nights max. In Vietnam, kids will love Hoi An (lantern making), Ninh Binh (boat rides), and Da Lat (adventure parks). A mid-trip beach stop (Da Nang or Mui Ne) could be a good recharge. Enjoy your trip

Thanks, with a kid and a long trip I think only 3 days at each location would lead to exhaustion very rapidly. Maybe I should consider a couple of three day stints followed by a bit longer and then some more 3 day places.

Are you traveling as a family of three including a 10 year old?

On your way to Siem Reap you can also make a detour in the Trat province in Thailand. There are some stunning Islands there around Koh Chang.

In Laos, do not miss Nong Qiaw. It was the highlight of our trip. In Cambodia, I suggest going to Kampong Thom. Prasat Sambor Prei Kuk temples are there. Though not as impressive as Angkor Wat, they are far less crowded and the walk through the jungle is wonderful. From there you can continue to Phnom Penh and then to Vietnam. If you love gorgeous islands, Koh Rong Sanloem is the place. It's a tropical paraide soon to be destroyed by massive construction. I highly recommend going there if you can fit it in your itinerary. I would also take a day trip from Siem Reap to Koh Ker.
To reach Siem Reap you can take a train from Bangkok to the Cambodian border. It is fairly slow, but you can stop in Prachin Buri, and spend a day or two there to explore the Khao Yai National Park. If you are lucky you'll see elephants roaming freely. From Prachin Buri it's 2 hours to the border and from there 3 more to Siem Reap via shared taxi. The train to the border costs roughly 3 USD, and the shared taxi is 10 USD per person. So you'll save money and visit some off the beaten path places.

Like milosmitrovic3921, I was very glad to see Sambor Prei Kuk and Koh Ker. IMO, each is stunning and each is very different from Angkor Wat or anything else you would see in the vicinity of Siem Reap. I visited both from Siem Reap. IIRC, Sambor Prei Kuk was about 3.5 hours each way from SR. I combined Koh Ker with Preak Vihear (another outlying ruin that I highly recommend) for a VERY long, but very rewarding, day trip from SR.

is there a summary? I presume you've done the trip now.