cwhitt I am so glad to hear that you are aware of the danger that co dominant trees represent. By the time your grandchild is old enough for a tree house the tree could be degraded much more. Once the fungus is in the tree there is no cure. I have cabled a few trees in the past and it is only a temporary fix. The tree is constantly growing and the rotted area will only get bigger as the tree gets larger and heavier.
Cables? It will be constant cabling as the tree grows and the weight of the tree continues to shift. The co-dominant area is very low to the ground . By adding a swing and a treehouse will add more weight and movement/pressure to a already diseased/risky tree . Leaving the tree as is is of course your decision.
Have you tried calling a certified arborist for a qualified opinion, and you would have the peace of mind on what is best in your case.
Or ..Plant a larger ...15' tree now and then you will have a safer alternative for this grand child and more to come in the future.
More info from arborist in Oregon - dangers of co-dominant trees.
https://youtu.be/9dDcdsskuN0
