I used to live in Phnom Penh - without children, but just a few ideas to get you started:
December's great for avoiding the worst heat, humidity, and rain but you may see prices a little higher and popular sites a little more crowded. For Siem Reap/Angkor Wat, you want to be there in "tourist season", which runs from mid-Nov to late Feb. I'd book a driver/guide ahead for Angkor and surroundings and make sure to let them know the children's ages - the ruins are great for kids but it's best to build in ample rest and food breaks. Also look at Tonle Sap for a relaxing evening/sunset cruise on the lake. If you're in Laos at the very end of December, you might be there for Boun Kin Jiang (Hmong New Year) - in or around Luang Prabang is a good place to be for it, and very fun for children.
Phnom Penh is a BIG city, and can be a little overwhelming. Traffic is crazy! But you're right, Garden City Water Park is legendary (there are a few others but I think this is the most well-regarded one). You might also like the Phnom Tamao Wildlife Rescue Center about an hour outside the city - ethically run, and has child-centered tours and activities. And keep Kids City Asia up your sleeve for a too-hot or too-rainy day; it's a huge sports/entertainment complex with loads of activities for kids.
For the adults - there are a few places related to the genocide, in particular Tuol Sleng Genocide Museum (S-21) and the Choeung Ek Killing Fields which are very worth visiting if you are interested, and helpful in gaining a clearer view of Cambodia as a country, but these are def not places for children under 12 or so. Also the National Museum in PP centre is amazing, especially if you're into archaeology - it's well respected internationally and they get some great visiting exhibits. The sculptures and the garden are cool for young children but some of the (extensive) exhibits may be a bit too much! I would avoid the riverside after dark with children (it's fine during the day) and also the night markets which can be overcrowded and hectic - much nicer ones in Siem Riep (although touristy) and Luang Prabang.
You mentioned beach trips in Europe, not sure if you want a few days at the beach as part of this trip but if so I'd look at Kep or Battambang (check out the Bamboo Train/Norry and local Tuk Tuk tours for kid-friendly activities). Kampot is also interesting although technically it's riverside not seaside, and Laos has some nice mellow riverside beaches too. I'd probably avoid Sihanoukville, which had a great reputation in the past but is overbuilt and a little bit seedy now.
I don't know Laos as well, but I'd recommend Luang Prabang and maybe Kuang See (waterfalls, water sports, and a kid-friendly bear rescue centre) for a start. And it may be too far out of the way, but take a look at the megalithic site Plain of Jars near Xiengkhuang to see if it appeals.