How to prevent sites to access to other site folder?

please help me ..
I'm working on an asp.net application have a main site and sub sites like this :
site.com/parent
site.com/parent/child1
site.com/parent/child2
Any page on parent site can access to child1 and child2 files and copy, delete them
but the problem is : child1 also can access to child2 files and copy, delete them !
I want to prevent child1 site to access to child2 files
I am using asp.net and IIS 7.5 and only have an asp.net shared hosting
please help !
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