class ChildCategoryHasItsOwnPageValidator extends ConstraintValidator
{
    protected $doctrine;

    public function __construct(RegistryInterface $doctrine)
    {
        $this->doctrine = $doctrine;
    }


    public function validate($value, Constraint $constraint)
    {
      //...
    }
}

PlacasFrontendBundleEntityCategory:
    properties:
        ownPage:
            - PlacasFrontendBundleValidatorConstraintsChildCategoryHasItsOwnPage: ~

/**
 * @Annotation
 */
class ChildCategoryHasItsOwnPage extends Constraint
{
    public $message = 'This category has a child category with an own page. You can not define an own page for this category.';

    public function validatedBy()
    {
      return "child_category_has_its_own_page";
    }
}

PlacasFrontendBundleEntityCategory:
    properties:
        ownPage:
            - PlacasFrontendBundleValidatorConstraintsChildCategoryHasItsOwnPage: ~

PlacasFrontendBundleEntityCategory:
    constraints:
        - Callback: [validate]

use SymfonyComponentValidatorExecutionContextInterface;

class Category
{
    public function validate(ExecutionContextInterface $context)
    {
        $children = $this->getChildren(); // assuming you have this relationship and its getter
        foreach ($children as $child)
        {
            if ($child->foo != '') // or whatever test you want to make
            {
                $context->addViolationAt(
                   'foo',
                   'This category has a child category with an own page. You can not define an own page for this category.',
                   array(),
                   null
                );
            }
        }
    }
}