First a disclaimer - I don't have kids, so I can't answer specifically how to help someone else, especially a 9 year old. However, I do experience sensory overload so I'd like to share some of things that I do to help myself (with the caveat that sensory overload is a spectrum, different things will be triggers for different people, and each person has their own tolerance of individual things as well as their own strategies that help them).
- I try to limit the amount information I have to process at once. I wear ear plugs most of the day, have a baseball cap that I pull down low, walk/sit/stand in the shade when possible. I try to zig when others zag, avoid crowded areas. I choose seats that give me a buffer from sensory impact, usually in a corner/on the edge, away from TVs/loudspeakers, and with less movement or chaos around me. Eating off-crowd time helps too. Getting to the parks early, and then doing things just before close also helps to be at attractions with less crowds.
- I plan around my limitations. I know I will need breaks. I pause between activities and rides looking for a quiet place to sit to de-compress. I know I'll have to leave the park right around lunch and zonk out in my hotel room for a few hours before coming back in the evening (and sometime not being able to return). I plan where I stay with this in mind, and pay more for lodging that will be quiet and give me that space. I know I have to initiate the pause and the break before I think I'll need it because if I don't it's complete breakdown and much harder to recover.
- I use
DAS. This is a lifesaver because it helps me limit that overall input and gives me the opportunity to pause. I don't try to do other attractions while waiting in line, even though Disney says you can. That would stack too much for me. I look for quiet places to rest, or grab a bite to eat for more fuel. Sometimes I end up returning way past my return time because I needed that space before I was in a place to be able to actually enjoy the ride.
- I watch point-of-view videos of rides/attractions before I even go to the park. This helps me completely avoid something that I know will be problematic and most likely a day-ender. Once max overload hits there's not really anything that I can do about it besides leave, go to a quiet place and completely re-set.
Sensory overload can be very exhausting because of the amount of energy and effort it takes to sort through and process all of that information coming in. When I was younger (and still today), I also had a lot of difficulty to monitor my own energy, emotions, and mood throughout all of that and was not able to recognize when I needed to pause/take a break or have strategies to manage it. There was always things I wanted to do but didn't have the self-awareness to know that either I couldn't or needed a strategy to help me do it. This can be really frustrating as a kid. Having a good plan to help your daughter manage the input, expectations, and the need to break can go a long way. Also listening to her and her needs is important - I have family that still don't understand sensory overload and keep trying to push more and more and get mad at me when I initiate a stop or a break. I also think at that age letting her have some control over what you do in the park and where you go/when can help as well. She'll most likely naturally navigate towards those areas that are less overload and more enjoyable for her.