when evaluating safety, I think about 1. what's the worst that could happen? and 2. how likely is it to happen?
best case:
If she drops into the pit from a dead hang and has enough experience/air awareness to make sure it's a safe pit landing with the kind of force a 20ft drop is gonna have behind it - it's pretty much completely safe. I don't think this would be unreasonable for a kid with lots of tramp experience.
worst case:
but what else could happen? just a little bit of swing will displace her landing by a whole bunch - does the kid know that? catching a leg on the floor could be a horrible injury. she could peel from her pullup, start a flip and land on her head from 20ft... oops paralyzed.
Then I would weigh the risk vs. the benefit.. is the benefit of this climb/chinup/death drop worth the risk? It looks kinda fun, maybe you're confronting some fear of heights... I'm struggling to come up with more benefits.
Definitely not worth it to me - if you're (the coach) gonna do something risky, it should come with huge benefits and you should curb the risk as much as is within your power. This is something I would expect kids to come up with themselves in open gym without supervision.