what is one way to cut the children's overuse of smartphones?

Just take it from him forcefully and give time restrictions on its usage. There is no easy way. They won't willingly give it to you unless you enforce strict rules.

Nowadays, There is an option of applying child mode in almost every smartphone. It will help to decrease the time being wasted on the smartphone because of the unavailibility of the certain apps. Another method is to, Give them phone only when they complete some particular task, lets say , finishing 2 chapters each day.In this way, their studies would'nt be affected.

makinng them go out more and also letting them restrict their data and voice calling per month.

It's not your place to parent your nephew. If his parents are okay with him using the smartphone like that, there is really nothing to be done. As far as your own kids are concerned, the easiest way to do it is to either not buy them a smartphone or set very specific limits for use when they first get it. It's important to establish that precedent right from the start so you're not sending them mixed messages.

One way - just cut them! There is no better way, just do it.. Take it from their.. I don't have a children but sometimes I just want to take smartphones from hands of my brother's children.

Children tend to follow what elders do, so first parents should reduce their phone usage in front of children and spend more time playing and teaching various things to them.

makinng them go out more and also letting them restrict their data and voice calling per month.