It is possible to log into a second App Store account on an iPad. It's just messy if used for long, and you need to be very careful to track which apps were purchased on which account. They won't update, or let you know when subscriptions are due unless you are current logged into the right account Also, I would not be surprised if the iPad that you did it on had some account login issues later. Mine often confused be with which account It wanted permissions for.
It's been a long time since I've done this or heard of anyone else doing so. It used to be a common way for people outside the U.S. to get access to apps not available in their country.
Not recommended.
Also, it won't solve the problem. The App Store will not let you download an app that is not compatible with the iOS version on your device. It does not matter how you buy it. It won't download on that device. Given the age difference between the app and OS version, it is also highly unlikely it would work properly.
If the youtubekids.com site is not working in Safari, you could try other browsers. There are a few that are remote session on servers that might work. Puffin Cloud Browser might work. It's compatible with iOS 8 and later (according to the App Store). Some Versions of Opera might work as well.
Keep in mind that the apps actually run the browser on a remote server.
