Hi handyman.
First of all, nice idea; and yes, much easier for kids under about 10 to learn casting with these shorter rods.
Actually, I learned to understand the magic of the double haul many years ago from a guy in Austria who had a 6ft 2pc split cane fly rod and used just the tip of this to demonstrate to me how the double haul works. I had tried this for several years and then all become clear in just a minute or two.
There are casters who throw fly lines quite far just with a "rod" the length of a pencil by using double-haul (since there is not much flex in a pencil), and some even go farther and use their thumb as rod - not so good for fishing - but very effictive to demonstrate different haul techniques, because the all important thing here is the timing, and as soon as you once see it and feel it, it all comes together.
I would suggest though to put a stripping guide on these, unless you want to have these as 2-in-1 to set up with the proper rod and the short handle interchangable. Many years ago I did a similar thing and made a short piece of handle from an old spinning rod (with a stripping guide) for the tip of a regular 9ft 2pc 6wt; worked quite nice. Unfortunately, we dont have much water here where you could use such equipment, but again, a great idea.
And yes, of course you can build one like these from a kit, the question is: where would you get this kit from? I think, you just need the tip section of a regular fly rod. I know usually the tip brakes, but sometimes also the butt section gets broken or damaged, or you start with the tip of an older, not-so-much used rod, or you have a damaged grip section of a 3-piece; all these make great starters for such rod building projects. If you have a good tackle shop nearby, maybe you can ask to browse their "borken rod department". With any luck something should show up here. And usually you get such otherwise useless pieces for little money or maybe for free. Unless you set out with a very specific idea what the rod should be, you have lots of possibilities.
I think there are some companies selling shorter rods around 5ft now, especially the ones offering fiberglass rods, but these are usually the light line weights like #2 or #3 max. With the tip-section-rescue project you can do any rod you like, 4 or 5 weight for trout, or 6, 7, or even 8 weight for bass bugging or fishing for pike in close quarters. I dont know how they would do in salt water conditions, but I never heard of anybody trying such short rods here; the thing is, a one-piece rod is usually the strongest you can get, the ferrules or section are the weak points in most rods. I just dont know if its possible to handle bigger fish, lets say 10 lbs up with such short rods; but then I think to remember the late Lee Wulff quite regularly caught salmon of 20lbs + with 6ft split cane rods, so it can be done.
Best,
Wolfgang