1

The time you are travelling is the hottest time of the year. Is it too hot? Up to you. You can expect the temperative to be over 30 dregrees every day and possibly 40+. If you are used to heat and if you take it easy then it could be fine for you and your kids.

2

Cambodia can be hot, last year in November it was 32-34C for several days, which was hot enough for me, April will be even hotter, you may want to take some sunscreen factor 30+ for the kids.

3

It's going to be hotter than a $2 pistol.

4

You'll want to start your ruin viewing as early in the day as you can. Get there for sunrise and move on from there. You also may want to head back to the comfort of your air-conditioned room from 11-3 then head back out if you have the energy. Make sure everybody especially the kids, drinks plenty of water. Stay away from the sugary drinks. I found the slightly chilled coconuts to be most refreshing and tasty. They really are about the best thing you can drink in such a situation. They also cost the same as a soda-$1 and you get more quantity. A good wide brimmed hat would also be a good idea. Hopefully you will have been somewhere else that is hot before you arrive at Angkor so you'll be a bit more accustomed to the heat.

5

It's going to be very hot and humid in April with temperatures well above 30 every day, and even exceeding 40 if you are unlucky.
Depending on your kids experience of hot weather it might be too much for them. Personally, i wouldn't take younger children to visit the temples in that heat,

As has been mentioned, start your day very early, head back and relex ( or find some shade) between 11am - 3pm and finish off later in the afternoon. Keep very well hydrated and regularly top up on high factor sunscreen.
A tip we were taught was to wear a dampened bandana/ scarf under a wide brimmed hat, or have a headscarf that covers the neck which is regularly wet and wrung out.

There are a number of options on the coast with the surrounding islands, but a lot will depend on your budget and how far out you want to be. Couple of the bigger islands are about 2hrs boat ride, with a single trip each way per day. They vary from very basic wood huts, to luxury private resorts.
Another option is to head out of the main part of Sianoukville toward the far end of Outres beach where they are some very quiet and chilled out beach huts (in converted boats) which might appeal to the children.
There was a road being built last year, so it should be possible to get there without too much walking.

http://www.sihanoukville-cambodia.com/

6

It is really hot in April, very humid. You will need to put sunscreen more than 2 times a day especially for kids and umbrella would be good too. What I did last time was going to Angkor Wat then came back to hotel to rest then went out again in late afternoon. You can rent taxi for whole day, it cost $12-$15 depends on hotels.
Sihanoukville is popular beach in Cambodia, you can find peaceful resorts close to beach. I would recommend this resort because it is nice and quiet. They also have kid's club so your children can enjoy their time as well.

http://www.sokhahotels.com/sihanoukville/
http://booking.hoteltrip.com/hotel/10030014-11376915O.html