Hi,

"Requirement: Whenever item created in MasterList --> a new item should be created in ChildList (Condition =if the item doesnot exist in the ChildList. )"

Does that mean that you might have duplicates in the Main list?

The workflow would have to start on Item creation.

There should be something unique about each item

The workflow would then look at the child list for that unique attribute and if exists update the main item with the item ID of the child. If not there create the item and record the item ID and update the main list item with the item ID of the child item.

When an item is updated in the Main list a workflow configured to start on item modification is started and compares the main item to the child.  The workflow knows which is the child as it looks it up using the item ID on the main that was updated when the child was created.