Kimthong11,
We just got back from Siem Reap. Had a fantastic time. My LO is 8 months old and we also traveled with grandparents. We managed to do 3 big temples a day but it was slow going primarily because of the heat. (I think harder on the adults than on the baby!) We were going to do a farther out temple like Bang Meleay (sp?) but decided at the last minute not to because of too much time in the car but again, I think the constraint was the adults not the baby. We went the very easy route and hired a tourist van and driver that took us around. The van was air conditioned and very comfortable for five of us. We took the tuk tuks around town and loved riding in those but for the longer days, the van was perfect. If I was to do it again, I would have done the farther out temples with the van. Supposedly a lot of the roads out of town have been improved so the travel time is not as long as it may have been a year or so ago.
Thanks to the rest of you for all the Cambodia tips! We tried them all. Did not stay at the Shinta Mani but went there for lunch one day!