Change the age of the child on the iPhone SE to enable parental control?

to whom to write to change the age of the child on the phone to enable parental control?I set up a kid's phone like an adult. there were no problems with Samsung, but with the iPhone there were only disappointments !!!

Samsung is many times better than iPhone! Much easier and more convenient than an iPhone! By many criteria, the Samsung suits me more. For the first time I use an iPhone. For 3 months I have not got used to it ... I think I will sell the iPhone and back to the Samsung!

[Re-Titled by Moderator]