How to lock phone time setting?

I would like to know if I can lock “change time setting” on iPhone. I currently have limited screen time for my child but he figured out that he can change the time and he will get unlimited time. For instance, we have it set from 9 am to 7:30pm but once it hits 7:31 he would change his current time to 9am the next day. This is more for when he is sleeping over at cousins house. He needs his phone in case he needs to call us for emergency but don’t want him to text or call random people at 1 am in the morning. It’s not a problem when he is at home because phones are put away at 7:30. Thank you.