The frame is made with 7deg angles all around, with 10deg angles on the back (so it's harder to push over). Frame components are held together with epoxied mortise and tenon joints, and are made of red oak. The seat and back are maple. Finish is tung oil.
The prototype plans I found are here:
That link will take you direct to the slideshow containing the dimensioned drawings. From what I can tell MotherEarthNews is the original source, but there is nothing on the plans image to indicate ownership. Even if you don't build exactly to their spec, the plans are useful for establishing proportions. I lengthened the legs by a few inches and decided not to include a tray, since I intended it to tuck nicely under the end of the kitchen table.
I started building the sides of the frame. For this part I took a few extra minutes and made a 7' bevel gauge and a 10' bevel gauge out of some scrap stock from the Whitehall stems. These were invaluable; there are a LOT of 7 and 10deg angles to measure in this thing.
Once epoxy in the side frame joints cured, I made the cross pieces and used a carefully-cut seat to make sure everything glued together square. This was a difficult clamp-up job but I figured it out eventually. The lower cross-pieces were added afterward because it's just easier to do once the upper assembly holds together on its own.
The seat back is held in place using a butt joint and epoxy. I added epoxy fillets (epoxy thickened with cedar wood flour) where the seat back meets the frame for extra reinforcement. The fillets are as good as chiseling out a channel if you don't mind the look of them, and probably stronger. I had originally planned for this part to sit in a small channel, but I got a bit careless and forgot to leave enough extra wood on the seat back. The butt joint was really my only option aside from scrapping the whole seat back and starting over.
I finished it up by sanding first 100 grit, then 220 grit. Finish was 3 layers of pure Tung oil (from Lee Valley). This is a food-grade finish when cured, and looks fantastic on lighter woods. It also wipes clean with water and doesn't seem to suffer from water damage in the way varnish does. I use the same tung oil on our kitchen table and would recommend it for any project like this.
To keep tung oil surfaces looking good, remember never to wipe with detergents (wet cloth only) and rejuvinate 1x per year with a light 220 grit scuff-sanding and a very light layer of new oil.
Safety straps are sewn around the back of the frame and I added a middle strap to prevent the little tyke flying out the front.
And here's the finished product. He seems to like it!