It should work, se here: http://stackoverflow.com/questions/8261737/web-app-active-directory-user-roles-vs-user-isinrole
Check that Child Group is a securitygroup otherwise it might not work
On further investigation I can see that I had it around the wrong way. A child group is considered a member of it's parent groups (recursively) - parent groups are not members of their child groups.
Hmm, I did not see that when I read, but that is true.
Great that it works!
Let's say I have a group "Parent Group". It has a sub group "Child Group". If I have a user, who is a member of "Parent Group", why is User.IsInRole("Child Group") == false?
Is there any way to "Set Access Rights" on a page such that group rights bubble? I would expect that a user who is a member of "Parent Group" would have access to any page which is set to allow Read access to "Child Group" but this does not appear to be the case.