You could put a kit in both the bow and stern hatch, make a small air hole in the hatch cover and close her up. That way, the boat would be trimmed well (depending on kid size) AND you could do rolls... Just kidding!!

Seriously though, I have placed my 3 year old in the aft hatch (without the hatch cover) and put some extra weight in the fore hatch to trim the boat out. When my wife and I do this (my other two children have their own kayaks) it is ONLY on calm lakes or very protected harbors, the 3 year old ALWAYS has a life vest on, and my wife is very close. The big danger is if a capsize occurred. I have intentionally tipped it over with the 3 year old in the hatch just to let her know what it is like. Since she is a very good swimmer (lessons since she was just over 1) it was no traumatic experience to her. In fact, she wanted to do it again. Prior to doing the practice wet exit I had installed, and still use, float bags in the aft hatch.

: If the child were not present in the kayak, ballast would need to be placed
: where the child was so that the boat would trim properly. The only other
: way I can think of is a sliding seat for the adult that can be locked into
: different positions.

: Another major concern is boat width. My experiences with children in narrow
: boats have taught me that wide is good. Children love to shift side to
: side which makes the adult constantly adjust their position to keep
: primary stability. Most tandem boats are much wider. If you went with a
: 17-18 foot boat in the 28”+ width range, it will lag behind other single
: kayaks.