I'm going to assume that what jallman meant was the average adult doesn't know the difference between a "guilty" and a "no contendere" plea. I'm sure if it was explained to them, they would understand it just fine. All I'm proposing is that the same criteria be held to a child.
Put another way, exactly how would you determine if someone is old enough to be charged as an adult? Are you going with the subjectively decided age of 18? Is there something magical that happens at 18 that grants them full understanding that someone 17 years and 364 days old does not possess?