The finish of the steps is tough enough, I'd be surprised if your little ones could damage it. Also, it's not 100% smooth, so getting PPF to stick to the surface would probably be a challenge. As soon as dust/dirt finds its way underneath, it would at best look horrible, but mostly likely it would start to lift.
The position of the drop step is pretty perfect though - it's really easy/natural to step off the drop step right into the cab.
That said, there have been a handful of times I've stood on the top of the step as well, like to clean the windshield, and I haven't noticed any wear from doing so.