Ok, thanks for the info. Were you successful in creating her Apple ID? Which email provider is she trying to set up on her iPhone?
... and you are using Parental Controls to restrict what she can access ... correct? If so, I do know that you can restrict access to Apple built-in apps, which would include the Mail app, but it would be a "all or nothing" type of restriction. That is, either she will have access to the app, or not. If provided access, I don't think there is any restrictions on her configuring it. I could be wrong.
At this point, I suggest that you contact Apple Support directly for further assistance with this. To do so, simply select the "Support" link on the lower left-hand side of this forum's web page.