Without knowing how strong, how heavy, and how confident your daughter is, it is hard to be very specific, but I do have some thoughts on the subject of kayaks for children.

I built a kayak for my granddaughter but she is much younger and smaller than your daughter. It is very similar to the "Parr" on the Redfish Kayaks site. I have also had some experience with children in canoes and in recreational and racing kayaks.

My first observation is that I believe it is a mistake to add more stability. Children are lighter than adults and have a lower CG so they have more stability in a kayak than an adult would have. They also develop a good sense of balance more quickly than a lot of adults.

A paddler needs to be able to go faster than roughly 4.5 knots or about 8 km/h to get any benefit from a waterline length over about 15 feet. So unless your daughter is stronger than average, she would probably be slower in an 18 foot kayak than a 15 footer because of the increased wetted area. On the other hand if you go down to 14 feet, that is getting closer to the point where she will fairly soon be limited by the potential speed of the hull.

My suggestion is consider something close to 15 feet and keep it quite narrow.