The only way you can start witnessing some changes in your children’s behavior is when you assume responsibility. If you are always on the lookout for the best phone games and apps, and spend a lot of time face down on the screen, then your children will certainly copy you and do the same.
The best smartphone piece of advice that you can give your children is, actually, to show a responsible behavior yourself. You are their role model after all, so what you do, they will do. Thus, try to get them used to the scene of you without a phone in your hand.