-- Last Updated: Sep-29-12 2:59 AM EST --
As to the kneeling thwart, that wouldn't serve the purpose of a child's seat very well. Nor are they particularly comfortable for long days solo paddling because you can't sit on them with your feet out front for switch paddling.
I would get a wide cane seat and mount it astern of midships. In this way, you will be paddling solo with the bow properly forward, which will avoid the whole issue of whether the hull is symmetrical. You will also have more room between the center seat and the stern seat than you would between the center seat and the bow seat if you put the center seat forward of midships.
The wide cane seat allows you to solo paddle sitting in the center of the seat or to slide over near the gunwale for Canadian style heeled solo paddling. It also allows two kids to sit on the mid seat, or one kid to slide over closer to the gunwale to pick water lilies or whatever.
You will have to remove the center thwart if you are positioning the center seat just 4" behind center on either side. That's okay if you you are content to use a clamp-on portage thwart, which I always was.
Otherwise, you can put the center seat about 12 inches behind the center thwart and leave the center thwart in position. This is a little far back for "perfect" empty boat solo trim but still a much better trim than sitting in the bow seat backwards.
There are also removable center seats you can drop over the gunwales.