It is suggested that you first set up Family Sharing:
Family Sharinghttps://www.apple.com/family-sharing/Create an Apple Account for your childhttps://support.apple.com/en-au/HT201084
Family Sharing
https://www.apple.com/family-sharing/
Create an Apple Account for your child
https://support.apple.com/en-au/HT201084
Then, please refer to Apple's own information for setting up Screen Time:
Use Screen Time on your iPhone or iPadhttps://support.apple.com/en-au/108806
Use Screen Time on your iPhone or iPad
https://support.apple.com/en-au/108806
Screen Time Tips | Setting Up Screen Time | Content & Privacy Restrictions