One thing I can tell you about children and common sense: don't believe that they possess it until you see consistent evidence of this.
Being moms, many of us worry about this. "Does my child know, with every fiber of their being, that touching the lightsocket with a screwdriver--because they look made for each other,so it could happen-- is a dangerous thing to do?" An electrical outlet is a dangerous temptation to a growing and naturally curious child, so we put those horrible plastic safety plugs in them, even as we curse at how difficult they are to dig back out.
I was tempted by that strange set of little holes in the wall, when I was four. A cousin said "put the key in the hole in the wall". I did. Didn't take twice, that's for sure.
Our little Kiddo is four and curious too. I like that he's curious, glad that he wants to understand some of the existential stuff better than he does. He asks a lot of questions about everything: dinosaurs, trucks, pipes, bees, cell phones 'dying' (what does that mean when non-living objects 'die'?), whatever's on his mind. He's wandering all over the yard, picking up parts of plants to put in some creation made of rainwater, mud, and crushed sidewalk chalk he and his friend call "ointment". He's starting to become an equal in both initiating and following the play with his friend, and I'm excited for him.
But with this comes other concerns. Now that he is more easily led by peers, what does that mean in regard to his personal safety and common sense? Just as I was the kid with zero logic whatsoever when it came to electricity, so might he decide to do something equally dumb. This is why I'm still careful to keep the less-safe items invisible. Putting away my iron today, I noticed that I automatically hid it behind my sewing machine, under the blanket that covers the machine itself. Out of sight, out of mind. In sight, something to explore. His peers won't likely be up in my room, but the idea remains that while he is relatively safe, he's not always making his own bodily safety a consideration. Hence, the hidden items. The oven knobs that are removed and place in a bowl at the back of the counter each and every time. Or the matches, stored up high out of reach. It's all I can do to tippy-toe to reach them.
Included in this is the idea that my kid still doesn't always know what goes where or that certain everyday objects are indeed delicate. Thus, we have to guard "precious" items. Our vintage kitchen table has a porous top, so we had a piece of glass made to cover it and use an oilcloth over that. Works great and Kiddo doesn't have to hear "use a coaster" a jillion times. He can play in his room but coloring and artwork belong wherever I'm working, because he will occasionally write on things he shouldn't. He wrote on his little wooden chair with colored pencil and we decided that he could have the chair back in his room after the scribbling has been sanded off. Yes, it's his chair, but we don't write on chairs and we must make it right. We'll do it with him, and we also learned a lesson in allowing him to have colored pencils in his room. Of course, before this, we hadn't had any recent artwork adventures in the house, so all that to say, they're pretty unpredictable, those kids.
It's for these same reasons I don't let children have at-will access to scissors when they aren't available for art. I haven't forgotten my own four year old exploits, playing barber shop on myself and my little sister, then hiding the locks in a crayon box, convinced that because the hair had been hidden, no one would know. Crazy, that kid logic.
Looking back at my childhood, I think of all the times my common sense was taking a vacation in parts unknown. Ten years old, climbing a boulder in the Oregon Desert near Fort Rock while wearing flip-flops and slipping, gouging my foot open. Dumb dumb dumb dumb. Eight, pulling a twig in and out of the a campfire until someone else nearby doing the same thing burned my hand, leaving a raw wound which now is the tiniest of scars and has left me with a lifelong respect for fire. Three, maybe, and thinking "I'll pour the tea kettle" and grabbing the kettle full of boiling water from off the burner just the way I'd seen my mother doing it, scalding myself. Drinking a cup of surfboard wax remover the babysitter's son left on the porch. A whole cup? Must have tasted good....
That's why we keep the antifreeze in the garage, and why we keep the garage door closed.
So, back to my first statement: don't believe they have common sense until you know in every fiber of your own being that they do. When they are, say 25.