Yes, it's a good idea to limit their time with 'technology'....whether it is video games,tv, movies, internet, cell phone, etc. I think too much of this stuff re-wires the brain and can be damaging to people's mental state, especially children. While they are living in your home, then you should put down rules.
Encourage reading. Require them to spend time reading. Get them a library card and take them regularly. Reading engages the part of the brain that must learn to concentrate. It can be hard going at first but it gets easier. Reading out loud as a family, taking turns reading out loud, is a very good way to practice this level of concentration. And when they get into an exciting book (there are LOTS out there), their interest will be hooked and they will want to finish the story.
Also, encourage drawing, painting, and other creative activities. Hobbies are a great way to learn new interests. Usually, though, if the tv or some other form of technology is nearby, most people choose those over doing something that requires more mental energy. This is true for me, I know. I watched way too much tv as a child when I should have been outside playing.