Can apps be shared across different user accounts on the same pc? In particular, if I am a parent and download a paid app for a child - is it possible for them to share that with a sibling who uses a different user account to log in and with a different user profile?

If this is not possible at moment can you send request to Microsoft and app developers as this will help to make apps more affordable for families with many kids as we are able to use most traditional desktop apps this way where say one educational software can be used by different users with different user profiles to record their progress individually.

Apps can be installed on multiple PCs and user accounts

These days, people may use several PCs in the course of their daily lives, or share PCs with more than one person in their household. We want these customers to have access to a great Windows 8 experience on all of those PCs.

The licensing terms allow customers to install and use their apps on any user accounts on up to 5 PCs. Why only 5 PCs? Because we also believe in the value of the developer’s investments in their apps, and we want to protect that investment from abuse.

The above is a small part from the article here.

http://blogs.msdn.com/b/windowsstore/archive/2012/03/12/licensing-apps.aspx