Just started to use SilverStripe 3.1.2. The cms/framework is very powerful and gives me a lot of flexibility.
That being said i seem to have hit a relative big problem for my current project.

VirtualPages.

It seems as if i can only add Virtual Pages at the root of the SiteTree. When i try to add a virtual pages in a children tree node if the properties $allowed_children is not set then i can add virtual pages. Since i want to restrict users to only being able to create Page 1, Page 2 and Virtual Page by setting the $allowed_children property of the child tree node class.
when i create a page, at the children tree node, I get an error when selecting virtual page: "Page type "Page" not allowed as child of this parent page"

here is the current code:

static $allowed_children = array('*Page','CartusHolder','*VirtualPage','CartusPage');

if I set the $allowed_children = array('SiteTree') or don't set it at all, I am then able to create Virtual Pages no problems,

Any help would be appreciated.

the fix is to update the sitetree.php method validate and add an if statement.

look for this:
if(!in_array($subject->ClassName, $allowed)) {
...
}

and wrap it in a parent if statement like this:
if(!($this instanceof VirtualPage)){
if(!in_array($subject->ClassName, $allowed)) {
.....
}
}

public function validate() {
		$result = parent::validate();

		// Allowed children validation 
		$parent = $this->getParent();
		if($parent && $parent->exists()) {
			// No need to check for subclasses or instanceof, as allowedChildren() already 
			// deconstructs any inheritance trees already.
			$allowed = $parent->allowedChildren();
			$subject = ($this instanceof VirtualPage) ? $this->CopyContentFrom() : $this;
		
		

			if(!($this instanceof VirtualPage)){//UPDATED BY MARK using solution found on https://github.com/silverstripe/silverstripe-cms/issues/773
				if(!in_array($subject->ClassName, $allowed)) {
					
					$result->error(
						_t(
							'SiteTree.PageTypeNotAllowed', 
							'Page type "{type}" not allowed as child of this parent page', 
							array('type' => $subject->i18n_singular_name())
						),
						'ALLOWED_CHILDREN'
					);
				}
			}
			
		}
		
		// "Can be root" validation
		if(!$this->stat('can_be_root') && !$this->ParentID) {
			$result->error(
				_t(
					'SiteTree.PageTypNotAllowedOnRoot', 
					'Page type "{type}" is not allowed on the root level', 
					array('type' => $this->i18n_singular_name())
				),
				'CAN_BE_ROOT'
			);
		}
		//print_r($result);
		//exit;
		return $result;
	}

Could be that this was already raised as an issue on Github 8 months ago, with another possible solution.

https://github.com/silverstripe/silverstripe-cms/issues/773

Maybe you could have a look and possibly create a pull request? :)