A child table uses a foreign key to relate records to the parent table.
for example, lets say you had users and user groups.
To create a one to many relationship of User Groups to Users, the User Groups table would be the parent table, the users table would be the child table.
Both tables will have a primary key column which is an auto number column, the users table will a foreign key column used to relate the user to the user group. In this column, you will store the value from the UserGroups Primary key column for the group the user should belong to
UserGroups:
userGroupID - primary Key
userGroupName - text
userGroupID | userGroupName
--------------+--------------------
1 | Administrator
2 | Power User
3 | User
--------------|---------------------
Users:
userID - Primary Key
userUserGroupID - Foreign key - relates to UserGroups.userGroupID
userName
userID | userUserGroupID | userName
--------+--------------------+------------
1 | 1 | Jason
2 | 1 | John
3 | 3 | Adam
4 | 2 | George
5 | 3 | Tom
In the sample data above our users are members of the following user groups:
Jason - Administrator
John - Administrator
Adam - User
George - Power User
Tom - User