You wouldn't want to walk to that pool because of the distance, but you could. You'd basically need to take the trail from the tree houses, past the Grandstand pool, to the Carriage House, through or around the High Rocks Spring pool area to the Springs, then across the bridge to the Paddock. That's a long walk!
You've got several bus choices, though. The first would be to take the THV internal bus to the Springs, then walk across the bridge to the Paddock.
The second option would be to walk from the THV to the Grandstand bus stop, then catch any park bus, which will stop at the Paddock before leaving the resort.
Either one of these bus + walk options involve about 10-15 minutes of walking, which really isn't that bad in a Disney context. If you're not into the walking, you can wait at the Springs bus stop after getting off the THV shuttle, and catch the Downtown Disney bus. That's the only one that will stop at the Paddock after the Springs.
That said, the tree house pool isn't that bad. It's not very big, that's true. And the frog fountain doesn't do much to make it a "themed" pool. But it's still a nice spot for a relaxing swim. It was never very crowded when we were there.
