OWA not allowing child users

[Fabricio]

I have a child domain and the users in the child domain
can open their emails that reside on the root normaly,
but when trying to use OutlookWebAccess internaly or
externaly they are not allowed.

I tryed giving full access to exchage folder for those
users, I also change the settings of Default web site to
\. I also gave users the right to logon locally to the
exchange server.

Any ideas please help