Can I restrict outgoing calls to certain numbers on my child’s iPhone ?

Limit communication at any time: Tap During Screen Time, then select Contacts Only, Contacts & Groups with at Least One Contact, or Everyone. Limit communication during downtime: Tap During Downtime. The option you selected for During Screen Time is already set here. You can change this setting to Specific Contacts.

Set up Screen Time for a family member on iPad - Apple Support