i went to Angkor during march. was really nice. Try get a flight straight to Siem Reap. Theres nothing much in Phonm Pehn except the killing fields and the genocide museum, but its worth a look if u have time.
If you truly want to see the temples take a 2/3 day pass and so u can take ure time at all the temples. There are alooot of temples. Go to Angkor during sunrise... its amazing.
And be prepared, Siem Reap is a very hooottt and dusty place.
PS: dont give money to the kids no matter how sad they look.