Disclaimer: My comments are made in good faith and not intended to be soap-boxy, so please take them simply as things to think about.
I know you didn't ask, but you should also factor in the smart's weight, which is about twice that of a golf cart. So even if you could limit the smart's speed to the same as a golf cart, you're looking at twice or more kinetic energy being imparted to a pedestrian, animal or another vehicle/property in a collision. Your kids may be safer in the smart, but what about the other person or thing they may hit with the smart? You would be putting an 11-13 year old in charge of an 1800 pound vehicle in a neighborhood full of other children, adults, cars, and pets.
Also, unlike a golf cart, the smart has restricted visibility and some prominent blind spots, which would probably be enhanced at a child's height. This would likely increase the chances of an accident if your kids are not as tall as adults (which they may be, I don't know).
The smart also has certain electronic stability, traction control, and ABS features. If your kids aren't familiar with these features and how the car behaves when they're activated, your kids may react in unpredictable ways that could cause an accident.
Finally, you probably already have done so, but you should double check your local laws if you haven't. Contrary to some people's opinion, a speed-limited smart is not a golf cart. So even if the local laws allow a child to operate a golf cart with a special permit or license, it doesn't necessarily follow that they are allowed to operate a motor vehicle that can operate on highways, which is a very different animal.