As we have no guidelines on them, I can't be of assistance.
However, it would be considered an exterior change and prior approval would be required.
If they are the manufactured structures (called tree houses) we would consider them as play equipment.
If they were home built into the owners tree, we would still consider it as play equipment but we have no specific guidelines on that.
One HOA I found on the internet considers a tree house as an Accessory building.
Click here to go to their website.
Another Association simply says it needs prior approval (see section 14)
Prior to the Association making any rule, you should also check with the County/City permit and zoning office to see what rules they may have regarding tree houses.