I agree with everything EastBay1K said, but... I am someone who hates hanging around the airport. While everything he said is totally accurate, you should have plenty of time to have an evening and morning walking around Mexico City if you grab your bags and immediately head out of the airport to a hotel. How much you see will just depend on your family and how much energy you guys have.
Customs/Immigration lines can be very long at MEX, but I've never had any issues. I've also had them shuttle me through their electronic kiosks section, which I still don't really understand how/why, but if they offer it to you, take it. You'll be through in 5 minutes.