Thanks for the replies! I appreciate all of the ideas and you guys are spot on. The main reason to build this is the creation of memories. My middle daughter has mentioned a couple of times that she and the youngest are willing to help me on it. Like many, if not most of you, I have lots of projects that require my attention..... but these precious gifts are only around for a short period of time and the projects will keep coming. My first choice would be to construct something that they will be able to use even when they get older, like a swing being incorporated into the design. They may not be able to squeeze into parts of the building like they could when they were younger but they can sure sit there and swing while reminiscing about it...
Now to figure out how to get this done without breaking the bank!