You will only be able to keep your kids off smart phones if you live in a community not using smart phones much for example the Amish. I think parents of the one percent could also create such a community for their children if they are diligent and choose the right private school.
My 6 year old son will go to a summer camp where smart phones are expressly forbidden. Not that I give him a smart phone but at least the older children won't have smart phones, too.