Forcing them to do more "school" in their free time if they're already good students is possibly going to backfire and turn them against all school. They're likelier to start bucking against the real homework they do have, from school, if you require them to do more via a learning center. Please reconsider your approach here.
If you think they have too little homework, why do you think that? What's the basis for your feeling that way? Talk with their teachers. They may have what their teachers consider adequate and appropriate homework for their age, learning level and grade. Is it possible that when you were growing up, or in the area or culture where you grew up, kids were expected to do huge amounts of homework? Do you come from a family that had extremely high academic expectations and which created a lot of academic pressure that was considered normal? Back up for a moment and think about your own upbringing. As parents we often carry the assumptions of the way we were raised and use them as we parent our own kids in a different time and place where those assumptions may not work. I am all in favor of academic rigor, but it's worthwhile to talk ASAP with teachers and counselors at the school. If your kids seem bored with school because it's easy for them, and that's why you think they need more homework, the answer may instead be getting them into whatever programs there are for gifted students. And those progams do not necessarily mean more homework either.
You say they don't want to do extracurriculars but I would tell them that they must do at least one activity each. Be very clear that they get to choose those activities but they must do something that does not involve screens. It does not have to be a sport -- in the area where I live there is a lot of assumption that kids all do sports, but if your kids don't want to do a sport, that's fine. They could be doing an art class, community drama, dance, volunteering, Girl Scouts (it is never too late to join Girl Scouting; they do not have to have done it since they were little if they want to start now!), church or religious youth groups if that's your thing, music lessons or group music with other kids at "School of Rock" type studios (they exist!), Lego robotics clubs, ecology clubs (if they are in middle to high school there should be LOTS of clubs and activities they can do through school, or you can look at your county or city recreation center web sites).
Let them work with you to find what interests them--not what you think they should be doing. If they don't do what interests them it will just be mom forcing them to do things. But they should be doing something. Just not more "school" unless there's an academic need.
As for the iPhone and iPad, limit their time. Period. They have to earn any time that is not for school purposes. They use them in public spaces, not in their rooms. They have to maintain certain grades, do certain chores, whatever. Don't allow "all their time" on the devices. If they're carrying their devices everywhere they go, don't allow it unless it's needed, for instance, for them to phone you for a pickup after an event.