Design into 3D

Mortise and Tenon Joinery

Cutting mortises and fitting tenons with minimal voids
Traditional mortise and tenons, like most wood joinery are cut with saw and chisels (possibly with a suitable plane as well) and have sharp linear features which are not easily replicated on a CNC with a round endmill.
One technique would be to cut the mortise with the part flat on the wasteboard, and the tenon held at a 90 degree angle in a vertical fixture, rounding it to match the radius of the pocket used to cut the mortise. See the chapter on Fingerjoints for a fixture and the mechanics of doing thus.
An alternative which could be cut in a single setup would be to cut the tenon in two halves each laid flat on the wasteboard and rounding them using a cove radius endmill as used for the Radiused Fingerjoints of the previous chapter. The mortise would need to be similarly rounded to fit, or one could relieve the shoulder using a V carving.

Full Thickness Tenons

A more expedient option is to use the full thickness of the stock for the tenon thickness and to cut the mortise with dogbones or tees, possibly using a cove radius endmill or a V carving to round off the mortise to allow the parts to fit snugly. It is this option which will be explored here.
When the stock has a tenon cut while flat the tenons will have a rounded base which will need to be adjusted for when cutting the mortises:
Tenons cut from flat stock.
When cutting the mortises it will be necessary to cut T-bones or dogbones to allow for the square corners, and the longitudinal edges will need to be relieved, say with a V cut.
Mortises relieved with a V cut.
With code in place to make the mortises and tenons, it is a simple matter to use them in a suitable design:
Child's step stool made with mortise and tenon joinery.
Arguably, the proportions should be adjusted, since the current dimensions seem more bench-like and is a bit low for use.
Child's step stool with visible mortise and tenon joinery