Hello! I’ve just made a child account for my daughter but I don’t figure it out hoe she can actually use it on their iPhone. Please note that until now we shared the same ID account. Thanks!

See the following Apple articles:

Set up Family Sharing

Set up Family Sharing - Apple Support

What is Family Sharing?

What is Family Sharing? - Apple Support

👉 Make sure that each family member has a unique Apple ID

Make sure that each family member has a unique Apple ID - Apple Support

Download family members’ purchases on iPhone

Download family members’ purchases on iPhone – Apple Support (UK)

Add a family member to your subscriptions from Apple

Add a family member to your subscriptions from Apple – Apple Support (UK)

Share iCloud+ with your family

Share iCloud+ with your family - Apple Support

When changing to the new ID, there may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos. Then make sure full resolution photos are on the device. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. See the article below for other places that the old ID should be removed. See Winston Churchill's response in the below discussion.

 How to switch Apple IDs on your iPhone or iPad.      

How to transfer all files from one iCloud account to another.       

What to do after you change your Apple ID or password - Apple Support.