Thanks for the info guys.
I am setting up the system for both me, and my three five year old grandsons.
For me, I will route AC power to the tracks, and run nice post war, metal, AC engines.
But, when those kids come over, I will reroute DC power to the tracks, and let them play with and run cheap, DC can motor engines. If they drop or destroy one, no big deal, only a $35 or so engine. If the particular engine turns out to run OK on an AC track, then that will be great and I can switch the power to the AC.
All advice is welcome. I am not a purest, and this will primarily be a hands-on teaching tool for my grandsons. Get them hooked on running the many types and colors of the cheap DC engines, then teach them about hand operated switches, and then teach them about electric switches. At age 10 or 12, they can easily understand the concept of reversing polarity on a DC set of wires, and hopefully move on to AC trains.
I have seen far to many father's and grandfathers say that they are going to build a train layout "for the kids," and then co-opt it for themselves, with far too much detail and complexity. The kids end up totally bored, watching Dad nail down cork or solder wires, and end up walking away. ("Oh yea, the boys just love working on the trainset with me." NOT!)
Mannyrock
