Is there any way to assign stylesheets to whole sets of pages using is_page( )

Question

Assuming that you’re using parent page “photos” as a path.

It’s good practice (and required by WordPress Theme Guidelines) to use body_class() function for output different classes, based on current page or post. Just insert this function in your theme.

<body <?php body_class(); ?>>

This function will add parent-pageid-(id) class on every child page, cause every child page of “photos” will share same parent-pageid-(id) class you can easily add specific styling to them.

.parent-pageid-(id-of-photos-page) #content {
     /** Put your style here */
}
.parent-pageid-(id-of-photos-page) #content p {
    /** Style for paragraph tag */
}