Can’t move child Apple ID to another family

We started our family sharing on an old email and we are all moving to a new family. However I can’t move my daughters (under 13) - as I receive the email request and click on it, a page opens to say ,Invitation to Family Sharing

To accept, open this invitation in the Mail app on your iPhone, iPad, iPod touch or Mac and click the Get Started button.

To subscribe, sign in to iCloud on one of the following:

• an iPhone, iPad or iPod touch with iOS 8
• a Mac with macOS 10.10,

The issue is that I am opening the email from the mail app on an iPad running iPadOS 15.

Please let me know what I’m doing wrong and how can it be fixed. Thanks!