Tim, sounds great. A couple of things to consider: you know the kids will jump off of it, so it probably shouldn't be too high (don't want them to hit the bottom) or too low (they will pull themselves up out of the water on the sides) - over time weakening the platform (if it is attached to the tree rather than on pontoons). The tree would probably stay alive longer if you just pruned back some branches and built around the trunk. Good luck.
