Whenever both adults are in the car, I suggest 1 of you sit in the back with her & simply keep her entertained so she doesn't resort to fiddling with it.... Otherwise, just make sure it's tightened properly such that you cannot pinch a horizontal crease at her shoulder -- while I do not recommend removing the chest clip it is only a *pre* crash positioner (it's not intended to hold the child during a crash, but merely to keep the harness straps properly positioned: which is achieved in Europe without any chest clips
)
What I'm getting at is that she's still so young that given enough other stimulation it's likely she'll lose interest in this game
When there's only 1 adult in the car, then a soft plastic mirror would be an acceptable temporary solution.