I grew up in redwood country and lived a ways outside of town. I started climbing them when I was 4 or 5.
Big trees often grow close to each other. Finding two of them 15' . 7' across isn't that big, especially for a Sequoia.
The problem with the tall trees is that usually the branches start pretty high off the ground. Unless you have a climbing rig, you gotta climb a stump or nearby (smaller) tree and cross into the branches.
On the other hand, new growth trees abound. It wouldn't be difficult to find a few younger trees to use. Their outer bark is (for lack of a better word) "crushable" so I'd use wide straps and see how it worked out. If tree straps mess up the bark on a young tree, maybe look for some alders instead... then again maybe it would be fine. If anyone's hung from a redwood, maybe they can chime in about this?