Not the OP, but I would expect any nine year old of mine to do what we'd previously discussed and speak to the nearest CM at either the ride exit or in the gift shop. Said nine year old, these days, would also have a cellphone on her person, so could call me to check on why the ride was taking longer than expected (assuming I hadn't called her already myself to let her know that the ride had broken down). Even my more sensitive child could handle a little bit of worry/anxiety - it never hurt her to deal with the occasional wrench in the works.
If a CM is worried, I expect they'll speak to the child and she'll explain that I'm on the ride and she's waiting for me. The CM then might get on their radio to find out what the problem is with the ride, and reassure my child. No problem!
If a guest is worried, then I expect they'd speak to a CM about it and things would proceed as above. Most adults know not to speak to a lone child. But, in the event some adult did feel free to start a conversation, by nine my children knew how to tell when an adult was being overly familiar or inappropriate in any way. (We rode public transit a LOT. And dealt with special needs individuals a LOT.) If the child feels in any way uncomfortable, she knows she's to walk over to the nearest CM and stick close to them. Because, of course, before allowing the child to wait by herself, we'll have discussed all these scenarios.
Nine year olds are pretty darn smart, most of them.
And actually, my son was far more sensible at nine, than he was at 14 after his hormones kicked in hard. That's when he started doing things like forgetting his pants at school. I used to swear, I thought he'd lost 30 IQ points overnight!
(Caveat: Of course, not all nine year olds are mature enough to handle being unsupervised for any length of time. And parental preference always trumps logic. It's like when you make your kids wear sweaters because you're feeling cold - some things don't need to be justified!)