prebres:
I kind of hate to weigh in on questions like this as I don’t want to influence someone in the wrong direction. So I will just make a few comments for your consideration.
With your budget and described situation, either the Power Cab or Zephyr would be excellent choices.
Many of us really feel that a computer interface, using the free Decoder Pro/Panel Pro software, makes programming and fine tuning DCC locos so much easier, and fun. Don’t know it you will have a computer near your layout but if you do you could get the interface for either system and still stay within budget.
Understand that with Power Cab you can only have a total of two cabs, or throttles as we call them, and the computer interface will count as one of the cabs as long as it is plugged in. However, with Panel Pro, you can run trains from on-screen (computer screen) throttles so, in effect, you can have several throttles working at the same time.
Now you can do the same thing with the Zephyr, but with it you can also have other hand-held throttles plugged in along with the computer interface.
If you are not intersted in the computer interface, as others have mentioned, with the Zephyr you can add a cheap DC power pack to run trains. This allows that operator to run speed and direction only, which your young son would have fun doing. With this setup he would not be able to push any buttons that might mess up your system.
Another consideration is power. The Zephyr provides 2.5 amps, which is all the power that many small to medium layouts will ever need. Power Cab at 1.7 amps is a little iffy for the future depending upon your scale, total numbers of locos moving at one time, and whether or not you are running locos with sound.
I hope that this will help a little.