There is plenty to do............they can read, they can go out and play, you know get dirty and have fun, they could play games together........do crafts, write a book, I'm kidding on the book........but there is alot to do.............
You could always give them a time limit on the computer and tv.....but you would have to make sure it's enforced for both...........you could set up a webcam and record whose on the computer and for how long.......or a video camera in an area they are to be or not to be without them knowing......
It's tough for parents now a days...........my son makes sure his kids go outside EVERY day......well, except on rainy days, but then they read, play games with each other, and they do play video games.........
Good Luck and you'll figure out something, just listen to them and then set the rules...........you're the adult........hang in there...........
Oh, there is also camps for the summer and such too to keep them busy...