I think this is feasible, certainly the designs which allow access to the lock release mechanism are a real problem.
The other stuff seems to be mostly a result of the electronic locking design. Most of these use an electromagnet to push or pull a pin into or out of the appropriate spot to keep the bolt from being retracted. These type should never be considered secure if they are mounted, though even then someone with training can defeat them with a rubber mallet and a lot of practice.
But, mostly I would say don't keep your firearms loaded. I don't think most three year olds can rack the slide on a semi-auto, unless the spring is really weak.
Most of all, I would say that if you're going to have firearms, familiarize your children with them the way you would with knives. Most kids don't stab themselves with knives because they're taught to leave them alone. Same logic applies here.