Tweens who sneak screens are often navigating a gap between their strong desire for autonomy and limits they do not feel they had a voice in. The sneaking is a bid for control. Understanding this does not mean accepting it, but it changes the response significantly. Coming in with accusation will close the conversation. Coming in with curiosity will open it.
I noticed what happened. I am not here to punish you. I want to understand what you were thinking and figure out something that works better.