I think that children will be called what you call them upon first introductions. I’m a cheerleading coach, and we have [name]Bella[/name] and [name]Catherine[/name]; [name]Bella[/name]'s given name is [name]Isabella[/name], but her parents introduced her as [name]Bella[/name] so that’s what we call her. I think when the child is old enough to decide, it will be out of your hands.
To be fair, I’ve always been [name]Jamie[/name], but my ENTIRE family still calls me Jammers. So, even if the child is called something else by her friends, family (specifically parents and grandparents) still have free reign to call them whatever.
My SO’s parents didn’t want to their children to have nn. They just asked people to please use their childs full name. so if your son’s name were [name]Michael[/name] you would just ask teachers to correct other students and would ask family members and friends not to shorten the name. Once my SO and his sister were old enough to decide she decided on 1 nn (that she never uses except for her email address) he decided on not using a nn. So (going along the previous example) he would always go by [name]Michael[/name] and when people ask if they can call him [name]Mike[/name] he just firmly say’s “it’s [name]Michael[/name]”.
I think it’s completely doable provided that one remains polite and firm about the choice for no nn with everyone - siblings, parents and the children themselves while they are young.