Another workaround: Go back to the desktop and try AnyTrans ($39.99 for a single computer), a file manager for iOS devices that has an integrated downloader supporting 900 sites, including YouTube and Facebook. It'll transfer the videos to the iPhone for you over the USB cable. Even if you don't pay for AnyTrans, the download option remains and is free forever.
NOTE: Beware of the ad traps on some helper sites—ads that look like they should be the download button to get your desired content, but they are not. Also, depending on the ad network employed by the site, your own virus detection software may throw up some warnings. The more the developers of sites rely on ads they don't control, or resort to trying to get you to place something on your system as "payment," the worse off we all are.
To protect your channel from COPPA, you can download all videos in the channel to your computer for backup or re-upload them to other platforms. Since online websites do not support batch downloading of videos, and most of them have suspicious and unsafe ads embedded, thus, it is better to use a safe and reliable tool like Jihosoft 4K Video Downloader.