An open-cockpit double is a great solution to taking young children out on the water in a kayak.

You really have two questions you are trying to answer here. One is: What boat can I build to take the kids out in? The other: What can I strip-build? While both questions are answered by chosing a boat to build, it's not necessarily the same boat. You already have a good answer to the first question. Build the Mill Creek 15. It'll go together quickly and get you and the kids out on the water.