Fwiw, I didn't/don't watch my dd on her devices 24/7, she had access to net from being a toddler as we are a gaming and gadget mad household, and as she became older and understood rules more she was given more freedoms, she's never added anyone to FaceTime, Xbox live etc without checking with me first, she's never downloaded so much as a free game without asking first, even though she knows how to. She takes her phone upstairs for an extra alarm and also to feel safe if she got trapped in room due to fire or whatever but doesn't play on it. She doesn't really bother with any tech mon-Friday as she gets homework done first.
It's not that I'm a perfect parent, not at all, it's that dd knows I can check what she's up to, be that sites visited on iPad, call history, chat logs, videos watched on YouTube search terms in Google etc, that I get notifications if she downloaded anything or when she signs into certain services, (my phones just told me she's signed into YouTube) that purchases would not go un noticed because I pay for it. She's expected to be signed into her google account, and from there everything done online can be seen, search terms, websites visited etc.
She knows it's a privilege to be trusted and be allowed access to the things she is, and that she absolutely will lose those privileges if she did any of the things she's not allowed to do.
I can't say for certain that dd won't ever break one of the rules, but if she did, I'd be mad at her and she'd lose all the access she has for a long time. It would be online for homework only and because I'd be unable to supervise at other times, she'd have all devices removed once she's shown she can't be trusted. She says herself it's not worth the risk as losing internet access means losing access to Netflix and doctor who :)