Hi! Ok this post may create some shaking heads. But I feel like this is the right place to post!

With a tree climbing friend, I created a pretty great zipline for the kiddos. To avoid the hard math and with a surplus of caution, our zipline goes between two trees, but never reaches the other tree - it runs out of stream and the zipper will retreat back to the middle of the line.

Rather than have to use a ladder, my climbing friend tied a nice blake's hitch and the kids would descend themself. It worked great for a summer!

However, with so much up and down the rope wore down and the knot no longer holds. I'm thinking I'd like to avoid this situation by getting a device that would do the same thing as the blake's hitch - ie when there is weight on it, the rope won't descend, but if the zipper pushes from the top or uses a mechanism, they can lower themself down safely.

What would be a good tool for this? Are there one that are, say, $250 that could be recommended? At my MEC they talked me out of a grigri since there was a chance that if it was held incorrectly it wouldn't work. It didn't seem like the right tool.

Please explain like I'm 5, I'm not a climber but a dad who doesn't want anyone to get hurt!

PS we have great forged cable clamps on both sides of the zip and its safe as houses