Parental controls - Divorced Parents

Two parents can use parental controls, but it has to be done though Family Sharing. See this reply Can 2 parents control screentime - Apple Community

Possible solutions now start to get complicated and I don't have a lot of time to list possible scenarios and solutions. If you do not care about sharing past purchases, you could set up a separate account and use it for a family group with purchase sharing disabled. From: Add a family member to your shared subscriptions - Apple Support

"If you're the family organizer and you don't want to pay for family members' purchases and subscriptions, you can turn off Purchase Sharing [ How to share apps and purchases with your family - Apple Support ]. When Purchase Sharing is turned off, everyone must use their own payment method. When Purchase Sharing is turned off, family members can share subscriptions, but they can't share individual purchases like apps or books."

Note that a person can only be in one family group at a time. If you and your ex and offspring have no real interest in other family sharing features then you could set up a dummy account for doing this. Otherwise you could maybe figure out a way to make it work with your current family group. If your main concern is payment, disable purchase sharing so each person has to have their own payment method (note that this may cause issues if you then have to enter a credit card into a child's account!) Alternatively if there is still some trust, make sure others have their own personal balance to pay for any purchases they make and leave purchase sharing active. You could also have sharing days when you enable it for a while while you and your ex keep an eye on each other, then disable it, or whatever. :-/

Based on: How to share apps and purchases with your family - Apple Support and How apps, content, and subscriptions from Apple are billed - Apple Support

1. If anybody in the family buys or subscribes to something, Apple first attempts to charge the item to that individual's Apple Account (Apple ID) balance, if any.

2. If a family member does not have enough personal Apple Account balance, any excess will be charged to the Family Organizer's primary payment method (usually a card of some kind). The Organizer's personal balance will not be used for purchases made by other family members. If Apple cannot bill the primary payment method, the Organizer will need to make another payment method the primary method. With Family Sharing the Organizer always has to have at least one functioning payment method.

Also note: "Some purchases, including gifts, can't be billed to Apple ID balance and will be charged to the family organizer's payment method." "Some subscriptions might not be charged to Apple ID balance."