I was once told by someone I had a lot of respect for not to make threats to children you don’t intend to, or can’t, keep. If you say you’re going to tell her mum and dad and don’t then you’ve instantly lost the battle. If she’s bright enough as you say, and you’ve properly explained the why so she understands it, then a report to parents, calmly, is necessary. At the moment she has no respect for you. That needs to change and only you with the help of her parents can retrieve it. Love is about respect and care. Good luck. Ps I’m paying attention as I suspect I’ll have this battle re tablets etc before long!! ?