If it is not something that you do regularly, your routine is already broken. You've gotten a child ready to go, which early in the morning in not usually an easy feat, and you took her to your car, strapped her in, and probably heard her at least initially. And during that time, whatever her plan to do with that child was in her mind. You don't leave the house to go to work with an 18 month old child if you don't have plan for someone to watch that child.
And simply forgetting that you have children is not a realistic suggestion here. I have been at work for a few hours today, i am well into my regular routine. I remember I am a parent. I know exactly where my children are, I know who is caring for them. Killing a child and then saying oops I forgot doesn't fly in my book. I hope to see this woman charged, and I hope she does pretty serious time. This is no different than parents who forgot to take their kids were in the bath, or forgot to put up the pills so the kid wouldn't ingest them, or forgot to not murder your kids and stuff them in a freezer, or forgot I took my kids to the park, or forgot there is a street in front of my house.