a) Many places are still going in/out of lockdown or are schooling remotely and so their phone is the only way for them to communicate with their friends. Depriving them of social contact is incredibly unhealthy and harms their development.

b) For better or worse apps like Tiktok are a huge part of their culture and the popular dances etc are often known by everyone. Being the only child who is out of the loop can cause serious isolation.

Children are growing and making them not feel like they are part of a social group is incredibly harmful and can have permanent effects in adulthood. Giving them a phone but monitoring their activities is likely to be the least harmful approach.