If it automatically defaults to "Admin", this is because the record has not been shared yet, so no contacts are available to choose from. Members in the following statuses will default to "Me- Admin" and can either be checked in by the admin or by using the "Other" option and writing in the contact's name.
If the child is older, they may also check themselves in. Use the "Other" option and write in their name (in the case of self-check in or self-release).