There’s definitely charging ports throughout the parks. So even if you complete go through your battery, you can plug in for a few minutes and get charged up. Thinking about it, it can be a great way to take a break and kinda just sit and enjoy the Disney ambiance. But if you want to keep on the go, a portable charging battery is your best bet (obviously).
However, something else to consider is the storage of the phone (not to add to your anxieties lol). iPhones take great video and photos, but the dang things have very limited storage (even when maxed out to 128GB). While in everyday life that makes almost no impact, for Disney, it becomes a bit of a liability. But the good news is there’s ways around it. You could use cloud services, but if you don’t want to that, there’s this thing: https://www.amazon.com/gp/product/B00AQUMZRA/ref=oh_aui_detailpage_o03_s01?ie=UTF8&psc=1
Charges the phone, and gives you the ability to transfer your videos and photos off the phone and onto SD Cards and/or USB Flash Drives by way of a private wifi. I used it when I went and it worked perfectly. I would set the transfer overnight and by morning everything I shot on the phone was backed up and I could then clear space on the phone. The only thing that i noticed is files that are larger than 4GB had issues transferring (so you just need to pause/resume once you get close to the file limit).
The battery size on the one I linked to isn’t that great, but I was able to fully charge my phone once on it with the battery near-depleted, which got me through the day without a problem.