Casually asked what she was doing on her phone in her room with the door shut. She was being evasive and weird so I asked to see her phone.
What followed was a tug of war/pushing/shoving (her, not me) and I eventually took her phone away and made sure she knew it would be for the forseeable future.
I've checked it, there's nothing particularly bad on there that I can see on Insta/Whatsapp/snapchat.
Am I massively invading her privacy? She's quite a young naive 11 yo. I'm prepared to be flamed here, but I'm so conflicted about respecting her privacy vs keeping her safe.