If Parental Controls have been added in the child's device, you will not be able to buy the app. 


When you try to buy any app subscription, you will get a message to say that you are not able to buy this item.


You get another message to "Ask your parent". 

You are required to ask the Family Manager to add a Family Payment method by signing up for Google Play Family Library. 



To set up Family Payment Method: 


If you are the Family Manager in the family group, you can grant purchase approval settings for any member of your family.

Steps

  1. Open the Google Play app.At the top right, tap your Profile picture.
  2. Tap Settings and then Family and then Manage family members.
  3. Select the family member's name to whom you want to grant access.
  4. Tap Purchase approvals.
  5. Select the type of purchases that you require approval for:
    1. All content
    2. All purchases using the family payment method
    3. Only in-app purchases
    4. No approval required

You will receive an email whenever the family member makes a purchase, for approval. 

For more details click here - Purchase approvals on Google Play