Hi there,
We stayed 5 nights in Siem Reap and 2 nights in Phnom Penh. If you haven't booked your accommodation yet, I can highly recommend the Shinta Mani (
http://www.shintamani.com) in Siem Reap. It's baby friendly and the staff are very helpful.
In terms of visiting the temples, I would recommend going early (before 8am) as it gets very hot by about 10am. You also get to avoid the tour groups which tend to arrive en mass around 9am.
Siem Reap is considered high risk in terms of dengue fever and malaria, so we dressed Finn in long sleeved lightweight cotton clothing and were pretty vigilant with applying 'rid' on the parts of him that were exposed. A hat is also a must. He didn't get bitten at all so we must have done something right :-)
We hired a car with driver and a tour guide through our hotel and although not ideal, we just held Finn on our lap. The distances are quite short though and the driver drove pretty slowly. I would not recommend getting in a tuk-tuk with the baby as it is quite a bumpy ride.
We used the sling most of the time, but still brought along our Maclaren stroller. It was useful at night when we went out to dinner as Finn could sleep in the stroller and we put a mosquito net over the stroller.
Feel free to ask if you have any more questions.