My 20 ga. semi auto shotgun rests in the closet in its case. It's been there for almost two years and I forget how to use it. I could figure it out in a minimum of time but not in an emergency situation. My rifle is not easy to grab- in the same closet with the magazine somewhere close by and could be found with the light on and a bit of searching. My revolver is loaded in its holster in a box with the Glock, also loaded, on the bedside table. They would be in a small gunsafe if kids were ever in the house.
The semi auto handguns and the revolver were the HD choice because they were easy to grab, uncomplicated to load and use and I seem to get more range time in with those. The shotgun was decided against for HD because of my large dogs. Like kids, they get in the way of the blast and I prefer to have a good chance of ALL of us surviving a Bad Guy intrusion. In addition, they are easier to handle and move about than a long barreled gun in the close quarters of hallways, doorways etc. I am also vertically challenged at 5 feet two, not very strong, and need to consider recoil, maneuverability, speed of use (practice and familiarity here), and speed in accessibility.
She will need to initially take basic gun safety and handling classes and plan to spend ongoing regular range time with whatever firearm she chooses. She will also have to acquire a mindset that allows her to think pulling the trigger is an easy choice versus not hurting the bad guy and having her kids hurt or killed. Some women cannot get beyond that lifelong conditioning society gives us. In that case, even a baseball bat under the bed won't help her. But at least she won't worry about the kids getting hold of it. They will just play ball.