Use the Related To feature to allow friends or other relatives to check in a child who attends with more than one family or household. This is a common set up for grandparents.
First, ensure that the child is listed or added as member or their primary family, where they regularly live. If the child attends with another friend or family member occasionally, go to that person's profile page.
On the right side, under the family/household area, click the blue "Add" button in the "Related To" section.
From the dropdown menu, choose the relationship. Once the relationship is chosen, a search box will appear for you to enter the name of the child. Once the child is selected, use the second dropdown box to confirm the other half of the relationship.
Check the box to "Allow Check-In" if this person is indeed allowed to check the child in when attending.
The Child will now be listed in the primary how as a member of the family, but also in the Related To section for the friend or other relative.