Of course it is. It's involuntary manslaughter (that's if you actually did forget, and didn't do it on purpose like some of those people). Punishable by up to 8 years in prison. Don't want to go to prison? Easy: don't leave your toddler alone in a locked car.
"In most states involuntary manslaughter results from an improper use of reasonable care or skill while performing a legal act, or [...].
"Many states do not define involuntary manslaughter, or define it vaguely in common-law terms. Some jurisdictions describe the amount of Negligence necessary to constitute manslaughter with terms such as criminal negligence, gross negligence, and culpable negligence. The only certainty that can be attached to these terms is that they require more than the ordinary negligence standard in a civil case. With this approach the state does not have to prove that the defendant was aware of the risk.
"Other jurisdictions apply more subjective tests, such as "reckless" or "wanton," to describe the amount of negligence needed to constitute involuntary manslaughter. In this approach the defendant must have personally appreciated a risk and then chosen to take it anyway.
What is the standard of reasonable care in these cases? What specific actions are necessary to demonstrate that reasonable care has been taken? What risk have the people in the article deliberately taken that is not also taken by the majority of parents?