IF there is room in your dwelling another option might be to remodel a piano room with some type of acoustical treatment of the walls / ceiling to mute sound. I believe that other members of this forum have done this. I suspect that this would be at least as effective as finding any kind of "silent" acoustic piano. Besides, trust me, your boys will quickly learn to hate playing on an instrument that is, well, handicapped at best.
As long as you are going to be buying an acoustical piano, go for a grand piano - the action is generally better, as well as the sound. And if you get a good one, it should be the last piano you buy. Take the boys and have fun letting THEM try pianos, and let them participate in the shopping and choosing. Having your boys learn how to control the volume output is a good thing as it develops sensitivity and control of the keyboard.