First of all, I'll assume you're familiar with how to operate a door lock? Second of all, I'll assume you know how to say "We were doing something that only big people do, honey. I'll explain it when you're a little older."
Door lock? Omg, what a great idea. Too bad people are human and can on occasion forget that little gadget at say 1am. Second, I've got a newsflash for you mister lawyer (in training), a child's inquisitory nature would outdo many a court session, sometimes the "simple" answer only leads to more.
And actually to say, I'll explain it later when you are older is avoiding (not the same as being honest and forthcoming) the question posed by the child. So, you're kid might grow up not to be a liar, but when they avoid telling you anything, don't be shocked. My point was this, to make absolute statements of what you would or would never do in a situation with which you've had no experience is hardly what I would call an educated statement.