The modern technology has many impacts on the present generation. The millenia is filled with internet and technology. It certainly has it's own positives and negatives. Though parents controlling their children not to use smart phones has valid reasons, we can't completely remove smart phones from the current generation children's life. So, in my opinion, children should be allowed to use the smart phones with parents' monitoring.
First, as any other modern technology, smart phones also has many positives and negatives. To talk about negatives, the internet is replete with malicious contents and fradulent events which targets the new users. For example, in the modern era, the dark net has become a platform for luring the adolescents into all shady acctions like weapon trading and drug dealing. Young children are very prone to such attacks because of their incognizant stage of life. They are not aware of it's consequences in a long time effect neither on health by using the drugs nor about the addictivity it causes. As adults, parents understands these things and afraid of the potential traps in real world which leads them to completely forbid the smart phones for their children.
In addition to the harmful things from internet, the smart phone usage makes the young children to get addicted to it's usage in the forms of games and videos. For example, parents are in a need to hand over the phones to their toddlers in order to feed them. This kind of activities may give a short term relief but holistically it affects a children's body activities. They may get addictive to the colorful screens and not participate in any body activities like playing, learning and developing interest on other curricular activities.
Even though there are such negatives, there are equivalent positive sides to the smart phones. The children can be independent on thier own and at the same time parents can be in touch with them all the time. Reaching out a person is not at all an issue in the modern times because of the various mediums like WhatsApp, Telegram along with normal calls. And addition with that, learning is an essential usage of the smart phones. Students can watch the seminar or lectures anywhere anytime and be consistent with learning. Apart from school or college seminars, internet is abundant with external resources to learn and exceed in any field.
In summary, though parents know the negative sides of smart phones, they should also consider the positive things that provides to the young society. They should teach and warn their children about both pros and cons, and let them explore the advantages. And occasional monitoring on their smart phone usage would not hurt their freedom through which parents can also get some peace on their children safety over modern technology.