You can subscribe by going to Profile > Subscriptions > and then tap the Unlock Premium button. Once you subscribe with your Apple ID / Google Play account, you will have the first two weeks of Premium access for free. After two weeks, your Apple ID / Google Play account will be charged for the recurring monthly subscription.
Disclaimer: The information on our site is NOT medical advice for any specific person or condition. It is only meant as general information. If you have any medical questions and concerns about your child or yourself, please contact your health provider.