I'm a bit worried that the next term for you to define may prove a bit controversial. With the surge of interest in maker events like Maker Faire and Burning Man, the meaning of the term "hacker" has (at least it seems to me) expanded and evolved in like manner.
So, how would you describe "hacker" to a group of kids?
Enter your definition for "Hacker" into the comments field, below.
More importantly, we should not encourage our children to be criminals, which is what the term "Hacker" now implies. Those who think it is cool to steal or vandalize computer systems are just bad people. Calling them hackers elevates their crimes to a level that they are not worthy to use.
"Hacker" is like "Gay" in that it is a word that used to have a defined (and useful) meaning, but you cannot now use it with its original meaning because, as Erebus points out, it's been hijacked.
I know that "Maker" has come into fairly widespread use and it is descriptive, but it always reminds me of "gone to meet its maker" as in the Monty Python dead parrot sketch.
"Geek" is probably a good one but does have negative connotations - induced more by society than by the geeks though.
Enthusiast? Amateur? I don't know. I just think you shouldn't be encouraging kids to be hackers?
A Hacker would be one of strong technical adeptedness with delight in solving problems, overcoming limits, extending boundaries and building new things which technically may involve bending some rules. The hacker's personality revolve around the concept that freedom of creavity self expression is the right of any person who can think well, hence they tend to be anti-authoritarian to anything that tends to oppose such freedom(as this can prevent inventing good solutions), plus to they strongly belief in self voluntary mutual help rightly appropriating the thinking time of other hackers as sacred avoiding anything that leads to drudgery and boredom.
The term Hacker was created to describe individuals who could find ways of modifying existing technology, devices or ideas into new and more useful implementations of such. Since the criminal element hijacked the term, most of the Good Hackers are now using the term Maker. I was always proud to have the skills to do good hacking as it demonstrated my imagination and superior skill set. Good hackers never broke the law. Though we did bend it a little.
A hacker is one who creates something but isn't concerned with the final polish to it. This can be in just about any creative area. An initial prototype is often a hack. There are a lot of rough edges to the design but it shows that the idea works.
It seems to me that the term hacker comes from going at a problem with rough tools, much like using a machette or hatchet to make furniture.
A software hacker would leave out most error detection just to prove that the idea works with a restricted set of inputs.
An electronic hardware hacker would throw a bunch of parts together with a loose idea of how it should work.
An engineer would follow up by cleaning up the rough edges and making it reproducible and robust.
A 'cracker', one who breaks into things, would first be a hacker. You don't clean up the rough edges when you break into something. These are a subset of hackers without a social conscience.