Our plan is to buy a 'house computer'. Probably a desktop so that usage can be limited. I don't think there is anything wrong with kids that age to have monitored and limited access to a computer. The world is full of them, you can't get away from it.
This is my thinking as well. I think the computer that DD and #2 will be allowed to use will be a stationary desktop and they will have a certain amount of time per day to be able to use it.