No, I understand that argument (namely, that the death penalty isn't a deterrent because it's not actually factored into the calculations of those whose actions make them subject to it).
What you're suggesting is that my knowing that I might go to jail for 20 years (as opposed to, say, 10) would make me more likely to secure my firearms around my 5-year-old. I'm saying that would be a ridiculously trivial consideration -- so trivial as to not remotely affect my behavior -- when you realize that we're talking about the life of my 5-year-old.