The short answer to the original question is no... there is no specific way the kids have to build their robot.  The slightly longer answer is that they do have to follow the rules as outlined in the challenge document (number of motors, right types of sensors, etc).  The starter-bots at ev3lessons, and the ones that BuilderDud3 has are all great starting points.  If they decide to use one of those, they need to make sure they give the original designers credit (even though it will likely be modified) and be able to explain to the judges why they decided to use that design as a starting point (maybe they liked that it was compact, the attachment system, had the right parts, etc... anything to indicate that they surveyed a few different starter bots and made a purposeful decision on which one to go with)

My $0.02

-Frank