Buy two guns. Purchase weapons you and your wife are individually comfortable with as no one gun is going to be perfect for both of you. Go to the range together and take the kids. Get training for yourself and your wife and carry all the time. You can store the weapons in a bedside holster or safe that is easily accessible. If someone breaks down your front door with the intent of robbery, you won't have time to open the safe to get your gun.
Teach your children about guns and take them out to shoot. The sound and destructive power of the firearm in conjunction with proper gun safety education should keep them safe.
You have a very good post here dunerunner about purchasing a weapon for each of them. Plus teaching their children about gun safety. Have you been able to attend any classes for firearm training? That is the best thing you can do from the start.
If you do not mind me asking masslibertarian, how old are your children? I ask becasue I am a father of 5. Ranging in ages from 21 to 8. The reason I am asking is like dunerunner said teach your children. That is one of the biggest things you can do to help keep your children safe around firearms. I have read and learned that a child who is exposed to firearms is less likely to play with them. They learn to respect what they can do. Children are very curious and like to get in to everything. If you hide something they will find it sooner or later. When you treat your weapon as a big sercet your little one's are going to want to know what that big secret is. From the time my kids could handle a handgun or rifle they have been taught how to shoot.
As for purchasing a single handgun for both of you that is a had one. You both have different hand sizes, grip stength, and a whole list of different things. You should each choise something for yourselves. Not only is it better that each of you have your own weapon, it will provide back up to each other in cause you have to seperate from one another, like having to pull one of your kids out of his or her room if they get cut off.
As for weapon choices, every one had his or her favorites. A revolver is a good place to start. Your choice of a Walter P99 is a good choice. If it fits you well and you can shoot it well then good for you. Me, I carry a Springfield XD45 or a Sig P229 in 9mm. My wife like my full size Sig P220 in 45ACP or her Glock 19. It all boiles down to what you and your wife are comfortable shooting. Good luck.