I have accidently created family sharing from my child's phone and it is now saying that I need to leave this family sharing and join a new one. I created a new one form my phone.

Disband a family group

When the family organiser turns off Family Sharing, all family members will be removed from the group at the same time. If there are children under the age of 13 in the family group, the family organiser can't disband the group until they move the child to another Family Sharing group.

The family organiser can remove anyone aged 13 or older* from the family group. If you need to, you can transfer a child to another family.