The takings clause doesn't apply for someone convicted of a crime, but at the same time the Constitution prohibits the government from making something illegal and retroactively punishing people for it. That's called an "ex post facto" law, and is prohibited in Article I Section 9 of the Constitution. The Fifth Amendment requires the government compensate you for "taking" your property, and that applies even if the government merely destroys the value of your property (or forces you to do so). As one example, the Supreme Court just recently held that if the government temporarily floods your land (denying you the use of it), it is considered a "taking" under the Fifth Amendment, and requires compensation.
As for that 12 year old, she is more at risk from a swimming pool than from any assailant. Should we ban swimming pools in order to protect her? A single incident with 20 dead kids isn't even a blip on the radar compared to the number of kids killed by drowning each year. Policy decisions need to be based on reason and evidence, not on emotional reactions to one horrific incident.
Finally, you know nothing about me or my background, except that I happen to own guns. I have had good friends killed by guns, including one who was killed in a drive-by shooting. I've also had a good friend protect his own life with a gun (he's disabled and confined to a wheelchair) without having to fire a shot. I have had serious threats made against me, and I have had people trying to break into my home in the middle of the night.
My guns have never hurt anyone, and I hope they never do. I am responsible in storing them (they are all locked up and unloaded, except for the one in the holster on my hip right now), and they present no danger to anyone unless they make an imminent threat against me or my family. I practice regularly, and can generally hit only what I aim at, specifically so that I can reduce the chance of hurting someone accidentally. I have taught gun safety to others and hold people to its rules in an exacting manner.
The blood of those 20 children rests on the hands of one person, and one person only: the person who shot them.