If you forgot to add the relationship to a child for a contact, or that relationship has changed, you can change it at any time under the Child Profile.
Next to each contact there is a pencil icon that allows you to change the relationship. Note that you only have to change the relationship under one section (Parent, Authorized pickups, Emergency Contacts) and it will be updated in all sections.
As the relationship is child specific, you would need to edit any siblings in each child’s profile relationship with the contact (i.e. if you change a relationship to uncle on one child, that relationship will have to be updated on their siblings manually).