Reducing voltage

Our club lets kids run the controller for one of the lines when we do certain
events (like, right now, with Detroit Festival of Trees), but we could use a way
to forcibly limit the speed on the line that they're allowed to run.  One way
that's really easy is to just build physical stops onto the top of the regulator
so the dial won't turn as far in either direction, but that makes it difficult
to actually operate the dial and won't be much fun for the kids.  The other
solution I just came up with would be to maybe limit the voltage going into the
regulator.  And that's where there's a problem.  LEGO power adapters, for some
odd reason, output in ~9vAC, not DC.  Finding an AC-AC power adapter is
difficult at best, and the only two that I can find through Radio Shack (the one
place I knew of that _does_ sell them) run 9-13vAC or 18-24vAC, which is not
going to help at all.  They do sell an adapter that goes as low as 3v, but it
outputs in DC instead of AC, and I'm not having much luck doing a generic
internet search for adapters that output in AC below 9v.  Does anyone know where
I might be able to find such a beast?

Also, is it safe to run LEGO trains on a regulator that's putting out DC voltage
when all LEGO devices are designed with the expectation of AC power input?  I
know this is an issue that caused a lot of people to burn out the power busses
on the RCX 1.0, which then led to the outright removal of the built-in power
adapter port on the RCX brick (and probably directly influenced the transition
to a power port that's built right into a battery pack for the NXT).

Failing that, would there be a safe way to step the power down _after_ it comes
out of the regulator?  Say, running the power to a 9v TECHNIC motor and then
gearing the speed down a bit before having a second 9v TECHNIC motor act as a
low-power generator?  The motor bogeys have to get a few rest periods throughout
the day or they'll overheat, which makes me really cautious about hooking up two
9v TECHNIC motors and forcing them to run 8-9 hours at a time.