Without knowing the kids and what they like to ski and how well they read a trail map, I would say Beaver Creek because to me it is easier to navigate and to end up in the right location at the end of the day. Besides, if you are waiting for them at the bottom, you can gobble up the chocolate chip cookies. That said, I would consider the terrain they like to ski before passing final judgment.
By alone, I assume you mean you mean you will let them ski together without you and not really each one alone. I also assume that you realize walkie-talkies only work when someone has the capacity to use them and on a relative line of sight basis. I would expect, for example, if they are in Pete's and you are back in Vail Village, that they won't work so well. The folks running around the resort who are using them have the benefit of a repeater. You may also want to consider giving them a cell phone to use for that afternoon. Though, none of these communication devices should be relied on.