Hi Nirman,
the steps to follow are:
1. Create OrderEntity
2. Create ChildEntity
3. Create a lookup field in ChildEntity to OrderEntity
4. Create a currency field (Amount) in ChildEntity and add it to the form together to the Currency field
5. Create a Rollup field into OrderEntity, datatype Currency and add this field and Currency field to the form
6. Initialize some ChildEntity with a value
7. Observe the right behavior of Rollup field into OrderChild
The main thing is one Order : many Child, the lookup field should be into ChildEntity and not viceversa...
