If the mom uses a computer rather than an iPad, you can easily do this with Google Plus:
The child should be able to see exactly what the mom is seeing on her screen, while at the same time being able to hear her voice.
The problem with both parties using iPad's is that Apple's SDK prevents applications from capturing images of the screen outside of that application. If this is a necessity, then the only hope would be to jailbreak the iPad and find a solution that let's you do that.
The only other way is if you had an application which had both an e-book reader and screen sharing capabilities into it. The most popular e-book applications (iBooks, Amazon Kindle, etc.) don't have such a feature at this time.