You can start the conversation with other parents at kindergarten pickup.
Each grade at our school has a pledge that kids & parents can sign to wait until eighth grade to let them have a smartphone. This can be as simple as a shared spreadsheet or a dedicated site like https://www.waituntil8th.org
My kids are still young but from what I've heard from families with older kids is that holding the line gets increasingly hard as they approach 8th grade. You have to be prepared to socially exclude families that let their underage children use smartphones or social media, the same way you wouldn't invite a family that lets their middle-schooler drink alcohol and smoke cigarettes to your kid's birthday party.
While you can never get everyone to agree to anything, as long as your kids have a critical mass of friends who don't have smartphones then not having one won't make them an outcast.
