Do you have the User Guide/Manual?
That will be the first place to look up and find out how to set up the Parental Controls ("Mommy Mode").
Try this link if you do not have the manual:
https

/www.nabitablet.com/support/
Then I would encourage you to register her Nabi to get direct Parent support for further questions.
Here is the link to do just that:
https
/www.nabitablet.com/sign-in?next=/dashboard
Direct support is best because the vendor will be able to address questions and problems as the answers may be specific to your model Nabi and software versions.
Your Nabi may need some updates and the support folks should be able to help as necessary.