one time cost for subscription model

We have an app that was initially rejected because it accesses content through a WebView that has already been purchased elsewhere. It is a reading fluency application for younger children. Basically a parent/guardian sets up an account and then adds a certain number of "readers". This is all done through a browser, desktop or mobile. Then the iPad app simply opens our login page and allows the readers to login and begin working (parents do not use the app). We decided to use a WebView in an app in order to give the users a full-screen experience and to prevent children from browsing to any site they wish.

As stated, the app was initially rejected because it did not provide an in-app purchase mechanism to purchase access to our content. We initially wanted this app to be free, since the content is purchased elsewhere. My question is, does anyone know, if we were to simply charge up front for the app (one time cost) would that be sufficient? Or does this type of model absolutely require that we provide an in-app subscription purchase?

Thanks!

one time cost for subscription model
 
 
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