Congrats on the little fuzzballs!
I agree with Blacksheep about the rice pads. When I had kittens I stuffed old socks with rice and sewed or tied the tops. Every few hours I heated them up in the microwave, and if it is too hot you can cover it with a washcloth or another sock so they don't get burned. Make sure you put the heating pad or rice pads on one side of the box only. This way, if a kitten gets too hot they can move away from it. As they get older they won't need any extra heat, usually around the same time that they are ready to leave the nest to play.
As far as playtime, the kittens will let you know when they are ready to leave the nest and explore. It won't be for a while yet, but when they begin to walk and their eyes are open. They won't spend as much time huddled together, and they may begin walking the perimeter of the box looking to get out and investigate =]