I've traveled with Khmer families in Share taxis the kids are great no complaining. However I would simply just hire out a whole taxi and ask them to drive carefully.
1 Do not inflict your child on other pax. It's antisocial and not fair on anyone. You might love them, but nothing would piss me off more than 6 hours with someone else's little screaming darling.
2 Share taxi is ridiculous concept. Unsafe. cramped and a rip off.
3 Private taxi hire is cheap, reliable and honest. you will have ample room, flexibility and control over what is going on and where to stop. I hired one from PNH to REP for USD70 yesterday from V-Silk travel but there are a hundred of these companies along Sisowath and the equivalent in other places.
1 Do not inflict your child on other pax. It's antisocial and not fair on anyone. You might love them, but nothing would piss me off more than 6 hours with someone else's little screaming darling.
Not all children spend their time screaming and I have had some very enjoyable train and bus trips with others small kids for company in SEA and helped pass the time on a long journey
I cerainly agree with you about the share taxis (even with out a small baby)which why I recommended private hire

Thank you for your advice, I appreciate that it is well meant and particularly point number 3 is helpful.
However.....
> 1 Do not inflict your child on other pax. It's antisocial and not fair on anyone. You might love them, but nothing would piss me off more than 6 hours with someone else's little screaming darling.
Please do not assume that all babies are antisocial, that they spend all their time screaming. For one thing it takes a huge amount of energy to scream and I doubt any child could consistently spend 6 hours bellowing. I appreciate that you may have been on some long journeys with some very stressed and upset children and for that I am truly sorry for you.
In seeking advice on whether to travel by bus or taxi I was not looking for advice on how to parent my child or how to be a good passenger on said bus or taxi. If my child was inclined to scream then perhaps I would reconsider the bus. As it happens, not that you were to know this, my baby happens to be incredibly mellow and a wonderful traveller. She rarely cries even at home and when on a fourteen hour long haul flight she did not cry once - other passengers and air hostesses commented on her behaviour with high praise. On bus journeys in Madeira and Malaysia my daughter has gently fallen asleep while rocked by the rhythm of the transport or played quietly with a toy while sitting on our laps. But you weren't to know that.
My point is that in asking advice on how to travel safely in Cambodia I was not asking advice on how to travel with a baby. Your comment, while well meant I am sure, indicates a very sweeping and incorrect judgement about the behaviour of all babies. All babies are not alike and therefore having a baby does not exempt me from considering public transport or a shared taxi.
That said, you're quite right about taking a private taxi and thanks to all others who posted advice about the best ways to make our way safely to our destination.
Do have a great trip and your post (#13) is far milder than i would have been in the situation