Start kids with a single shot gun, it will teach them to aim well and to make the first shot count.
Consider replacing the recoil pad on an older shot gun. Old recoil pads get hard and ineffective.
I hope you will be always be there supervising your kids with a shotgun. Seven year-olds still enjoy shooting at each other with toy guns and they will shoot at each other with an "unloaded" shotgun too. Do they really comprehend that a gun is not a toy yet?
Why not start with a weak BB gun when they are mature enough to be responsible and safe with guns? You will loose enough windows, have vehicle damage, holes in the house siding and have other property damage with young kids and just a BB gun.