A two-year old going to daycare would need some sort of bag to go along with her - diapers, a snack, change of clothes, whatever. If the child is in the back seat, it's as simple as placing the bag with your purse/briefcase in the front seat as a reminder that you have the child.
If I thought I might get so distracted I'd forget my most important role in life, then I'd drive with a toy or something in my lap so that I couldn't leave that car without being reminded of my child. (this is a hard concept for me to grasp--in the little bit of time I spent away from my kids going to work they were always first in my mind)
Yeah, those are good ideas. But it doesn’t take away from the fact that its often just a terrible mistake when this happens.