Here's my take on your question. I think the easiest to start out on is the Sierra (Wall St. 2 at Woodcraft) because it is a single tube, a straight cut and very easy to assemble. Next I would suggest the Atlas (Polaris or Carberra) because it is still one tube, but introduces the kids to a cut that differs in width from one side to the other. It is also easy to assemble, but slightly more complex than the Sierra. The third would be the Retro (this is the Woodcraft Retro) because it has two small tubes but both straight cuts, and offers a little more challenge to assembly, but still easy. The Retro can be easily customized so that the kids could use two different or contrasting pieces of wood, making it more individual. All three are nicer to write with than a Slimline, IMHO. Also, these three are not expensive kits, which will be a concern. If kit cost is no object, a Baron or Sedona is an easy kit, but pricey. The kids might want a carry pen for themselves, so the Baron Sedona is expensive and the Cigar is a huge pen. I would highly recommend to keep the variety of kits down as they learn at first. They can learn different types of wood, then PR and the rest, without the need to branch out so much. Then throw open a Penn State catalog or Woodcraft website and let them pick what they want to do next.