C.W.
I'm going to be very blunt, not trying to be a b@#$%, just putting it straight. They don't want to do anything else because, they can...Meaning because you let them not do anything else, but this. The 7 yo especially should be trying new things, meaning sports, how does he know if he likes them if he doesn't try.
Kids sometimes need a "nudge" to get them going or motivated to do something else other than what they know. For example, I have twins who are 10 yo boy/girl. They are both very athletic, they both get one sport at a time and are in something pretty much year round, which we love. My daughter always did soccer in the fall, and these last couple of years you could tell she wasn't as into it as she always had been. Well, she reached the age she could do volleyball, well she didn't want to but we told her to try it for one season and if she didn't like it she could go back to soccer. Guess what, she loved it.
My point is, sign them up for things, don't let them quit once it's started, if they don't like it you'll know for next time, but get them out, get them active and get them away from the video games!!!!