Hi Ronit
I have a similar requirement where I have to allow user to input data at higher level - not only parent but at an aggregated level too.
I have achieved this using BPC and VBA. You can have base level data in one sheet and hide it. In the next sheet, you can display data at your desired parent level and allow input in that sheet. When user enters data, you can perform allocation based on your weights or business rule in the base level sheet. The data will be saved at the base level. Since user will see only data at the parent level, next time refresh will again present the correct numbers at parent level.
You can delight user by giving an option to dig deeper using base level sheet.
Other option, could be to create dummy member as parent and allow user to input there and then use Weight functionality or script logic to save data at the child members.
Hope that helps!