There is no good way to tame the high-strung nature of postwar 4- and 6-wheel steam locos. They're clumsy and tragically under-geared. Like most of you I started with one of these 2-4-2's and after a few broken marker lights (and scolding, and tears) I learned to control it. But it's really not a good design, there's no easy way to improve it. Limiting the voltage might limit the absolute top speed. But-- the behavior of these motors changes a lot with various loads. When you put several cars behind the loco, the lower voltage may not be enought to start the train moving, so it will just sit there with the motor getting warm. Not good!
A LionChief Plus Pacific like 6-81311 is the about smoothest and easiest-operating steam loco I can think of for O27 track. Synchronized sound and smoke, looks good on sharp curves with traditional rolling stock, not a lot of details to break off, kid-friendly wireless remote. Another key benefit... At some point in the future you might want to have two trains sharing the same track. (This makes for very exciting operation!) So get another LionChief Plus. There's no need for block wiring (which is a boon for temporary floor layouts) and unlike a block system you have true individual control. Heck, you wouldn't even need a second transformer! This is how I'd want my son to be introduced to the hobby!
If you're determined to stick with postwar, then I suggest a "boring" diesel like a 622, 2033, or even a 2383. 
These have a lower center of gravity and a much better effective gear ratio. A dual-motored 2383 will give smoother starts, more consistent running at slow and medium speed, with a more muted top speed than any of the small postwar steamers.
One more thing... O27 is a limitation. You don't have to go all the way to O42 -- that takes a lot of space and really limits what you can do on a 4x8. But, even starter sets come with O36 these days. If you're willing to step up from O27 to O31, the LionChief Plus 726-style Berkshire runs even better than the LC+ Pacific. Or... a 773! NOT kidding! It's the only postwar steam loco that runs like a diesel. With 18:1 worm drive, he can't run that one fast enough to leave the track. It's ironic that the most expensive loco Lionel sold after the war is actually the best toy -- more forgiving and less frustration than the entry-level models.
"It's the gear ratio, Brian. It always has been the gear ratio."
