For Google Play the requirements are:

• the family manager is over 18 and has a payment method on file (they manage the family wallet).
• the family members are in the family managers family, (and if under 13 the account is created by the manager).

I only have direct experience with managing a kid under 13, in that case I have created the account for him and never entered a payment method on his account. For any purchases he wants to make via the “family wallet” it needs my direct approval, which can be granted by using an app on my device or directly entering my password onto his. After either of us has made a purchase we have a “share with family library” toggle that can share the title with the other family member. Not that it only applies to direct title purchases from the store, if a feature is locked behind IAP it can’t be shared. We have his a could locked so he needs my approval for any purchases (including free apps) but this is not required by the platform.

For child accounts the family manager can choose between requires approval for each of th feollowing on each child account:

• All content
• All purchases using the family payment method
• Only in-app purchases
• No approval required

I presume the family manage ornly has control of the Family Wallet for adult family members but I don’t have direct experience to confirm…