I made this for the backseat headrest on a Volvo XC60 and an iPhone 15 with Apple silicon case — pretty specific, I know. 

I left the bodies un-merged in the Shapr3D file so that you can extend the length and adjust the ledge tolerances for various phones. The phone should fit very snugly in this holder, so that heavy braking won't dislodge the phone. I make no claims about the safety of this device, but it may just save you from a few headaches if your toddler doesn't enjoy long car rides.