Bruno, I hear you. When my son was little, our only car was a Ford Festiva, a two-door hatch-back. His seat was mounted behind the driver's seat, so the front passenger -- my missus or I -- would have easy access. Many was the time, driving alone, when I would have to look around because I forgot if he was with me.
In fact, more often that not, I would hear nothing, check, and then realize I had already dropped him at daycare.
Yes, but that's the point--YOU check. Bruno checks. RESPONSIBLE parents check, because there is always a part of them that is thinking about their kids. Constantly. I know I'm a hardliner on this, but NOT checking, and winding up with a dead kid as a result, is, IMO, unforgivable. And if your life has too many "complicated circumstances," you aren't prioritizing your kids. Parents who don't make their kids their priority are, by definition, lousy parents. Good parents who are busy, or forgetful, or harried, either make changes or develop failsafes to keep their kids safe. With young kids, eternal vigilance is the name of the game. If you're not up to it, you need to make some adjustments.