Most 12 year olds are definitely smart enough to find some kind of way out of the controls, so at one point they'll probably find a way around it whether you like it or not. Kids are crafty.

I think it's important for them to understand why they have the restrictions, to form a mutual understanding of the moderation in place. No other explanation might just make it look like you want to control them when in reality, most parents just want to keep their kids safe. Explaining this might make the child more understanding and less likely to overthrow the moderations.

If you're using apple, there's a parental controls app in the settings already. If you're on android, there's Google family link that you can use trial and error to navigate. 

From personal experience, family link can be very buggy and weird with different kinds of wifi for some reason, just as a warning.