I would be very pessimistic about the future value you can extract from any mining rig, even an ASIC one - you can't just linearly extrapolate.

Given that you want a father-son project I recommend building a computer with a GPU that can be used for gaming and mining with it when it is not gaming.  It won't make you rich, in fact it could end up costing you after power, but unlike an asic miner it has alternative utility.  Join a mining pool and you will start collecting a small pile of btc.