I agree with the other replies that you should see how you're daughter likes if before investing too much into it. A little fixed keel will help with the tracking and way less work than a drop down type. For hatches, you're back deck looks flat enough to put on a Beckson and on the front, you can put one off to the side of center. Some early plastic kayaks used to do that. Closed cell foam bulkheads are the easiest to put in. You can even use that 1" blue builders styrofoam from Home Depot but your fit will have to be a little more careful because it doesn't squeeze into shape.