How to determine efficacy of index on child side of foreign key
There are basically three situations where you would want to index the child in addition to the parent in an FK/PK relationship:


If you ever select from ITEM based on order_id (obviously!)
If you ever run queries on from the JOIN of ITEM and ORDERS with a filter on ORDERS
As with 2, but with a filter on ITEM that is less selective than that on ORDERS


In both cases, ITEM will benefit from becoming the inner part of the join and an index from on the child will help support a loop join strategy.