you can add child items to an item at the time of item creation.
1.Goto Master > Item > then Click on New button create the required child items
2. At the time of parent item creation fill the information of the item and then select the child item section. (If parent item already exists then you can edit that Item)
3. Click on Child item tab select item in item field and in the Quantity field enter the quantity of that child item which you want to add.
4. Click on the Add button.
5.If you want to add more that one child item then repeat the step 3 and 4.
6.After adding the item click on the OK button.