I would recommend building a canoe as your family craft. It can carry all four of you in comfort. The three year old can sit in the middle the 9 year old and one of the adults can switch off.

Ask yourself this question. How far or how long is a 9-year-old going to want to paddle? Probably not nearly as far or a long, as you want to go. The canoe solves this problem by putting the 9-year-old in a boat with an adult. If you still want a kayak, build two, one for the wife, and one for you. Then paddle them on those nice weekend getaways.

As the 9 year old matures, gets stronger, and gets a longer attention span (trust me it will happen some day I have a 14 and an 18 year old) you might need to build new kayak. But by that time you will be itching to build a new one anyway.

Seriously, in bicycle safety classes we tell parents to not let their children ride in the street until they are at least 10. This is because children under ten (and even older) lack the strength, coordination, and the skills recognize trouble or to get themselves out of it. This is even truer on the water. If your 9-year-old capsizes in a single kayak, he or she will most likely not be able to re-enter the boat on his or her own. If you are in a kayak also, you now have to deal with your child, your child’s boat, and your boat.

Hope this helps

Don