I haven't been to Pnom Pehn, but if you're going to Siem Reap my advice is to get a nice hotel with a good pool as temple visiting can get hot and sweaty and it's worth it to have somewhere to get away from it all at the end of the day. We went for a lower priced place and regretted it (it was the Golden Banana so that should give you an idea of the kind of price range I mean). We had problems there with power cuts, overflowing drains into the bathroom during storms and aircon not working in rooms - I would definitely pay a bit more and stay somewhere else next time.
If you take your children, take their passports to buy the temple tickets as they get cheaper tickets but they told us that they would need proof of age (we didn't have ours so had to pay full price).
The temples are nice but tiring - the jungle temple with the 'cheese' trees is one of the most exciting for children I think. Angkor Wat is good but there are loads of other temples around there to go in if Angkor is too busy and the temple ticket gives you access for the whole day (you can buy more than one day if you want).
Siem Reap is quite a nice town really but very busy and you get hassled to have a fish spa, take a tuk tuk, eat a meal, buy a painting.........almost everywhere.
There are loads of restaurants too - you can get everything from Pizza to Mexican to Indian and most of them are quite good in our experience. My recommendation is the Blue Pumpkin near the old market with the aircon lounge for a nice ice cream treat for the children (and for you too - they do ice creams with things like real rum poured over
).
Prices compared to Singapore are cheap too - good food for little money.
