My role would be to talk to them about the importance about playing outside, knowing nature and learning practical things. Playing with gadgets can give instant fun, but too much of it can spoil your eyes and concentration. They must understand that the best years of their lives should not be spent just pressing the buttons and have fun. There will be long term effects if you do not go out and get some exercise and there is no point in regretting afterwards.
My approach would be to slowly encourage them to play outdoor games , without immediately banning the gadgets, because if something is taken away forcibly from someone, they would crave for more of it. The point is to slowly and steadily attract the towards outdoors so that they start loving it and want to do it themselves, so that this practice stays with them through out.