diff --git a/wagtail/api/v2/endpoints.py b/wagtail/api/v2/endpoints.py
index b045140ce..6474ec266 100644
--- a/wagtail/api/v2/endpoints.py
+++ b/wagtail/api/v2/endpoints.py
@@ -18,7 +18,7 @@ from wagtail.wagtaildocs.models import Document
 
 from .filters import (
     FieldsFilter, OrderingFilter, SearchFilter,
-    ChildOfFilter, DescendantOfFilter
+    RestrictedChildOfFilter, RestrictedDescendantOfFilter
 )
 from .pagination import WagtailPagination
 from .serializers import BaseSerializer, PageSerializer, DocumentSerializer, ImageSerializer, get_serializer_class
@@ -189,8 +189,8 @@ class PagesAPIEndpoint(BaseAPIEndpoint):
     base_serializer_class = PageSerializer
     filter_backends = [
         FieldsFilter,
-        ChildOfFilter,
-        DescendantOfFilter,
+        RestrictedChildOfFilter,
+        RestrictedDescendantOfFilter,
         OrderingFilter,
         SearchFilter
     ]
diff --git a/wagtail/api/v2/filters.py b/wagtail/api/v2/filters.py
index dedb69813..dc989130c 100644
--- a/wagtail/api/v2/filters.py
+++ b/wagtail/api/v2/filters.py
@@ -110,42 +110,73 @@ class SearchFilter(BaseFilterBackend):
 
 
 class ChildOfFilter(BaseFilterBackend):
+    """
+    Implements the ?child_of filter used to filter the results to only contain
+    pages that are direct children of the specified page.
+    """
+    def get_page_by_id(self, request, page_id):
+        return Page.objects.get(id=page_id)
+
     def filter_queryset(self, request, queryset, view):
         if 'child_of' in request.GET:
             try:
                 parent_page_id = int(request.GET['child_of'])
                 assert parent_page_id >= 0
+
+                parent_page = self.get_page_by_id(request, parent_page_id)
             except (ValueError, AssertionError):
                 raise BadRequestError("child_of must be a positive integer")
-
-            site_pages = pages_for_site(request.site)
-            try:
-                parent_page = site_pages.get(id=parent_page_id)
-                queryset = queryset.child_of(parent_page)
-                queryset._filtered_by_child_of = True
-                return queryset
             except Page.DoesNotExist:
                 raise BadRequestError("parent page doesn't exist")
 
+            queryset = queryset.child_of(parent_page)
+            queryset._filtered_by_child_of = True
+
         return queryset
 
 
+class RestrictedChildOfFilter(ChildOfFilter):
+    """
+    A restricted version of ChildOfFilter that only allows pages in the current
+    site to be specified.
+    """
+    def get_page_by_id(self, request, page_id):
+        site_pages = pages_for_site(request.site)
+        return site_pages.get(id=page_id)
+
+
 class DescendantOfFilter(BaseFilterBackend):
+    """
+    Implements the ?decendant_of filter which limits the set of pages to a
+    particular branch of the page tree.
+    """
+    def get_page_by_id(self, request, page_id):
+        return Page.objects.get(id=page_id)
+
     def filter_queryset(self, request, queryset, view):
         if 'descendant_of' in request.GET:
             if getattr(queryset, '_filtered_by_child_of', False):
                 raise BadRequestError("filtering by descendant_of with child_of is not supported")
             try:
-                ancestor_page_id = int(request.GET['descendant_of'])
-                assert ancestor_page_id >= 0
+                parent_page_id = int(request.GET['descendant_of'])
+                assert parent_page_id >= 0
+
+                parent_page = self.get_page_by_id(request, parent_page_id)
             except (ValueError, AssertionError):
                 raise BadRequestError("descendant_of must be a positive integer")
-
-            site_pages = pages_for_site(request.site)
-            try:
-                ancestor_page = site_pages.get(id=ancestor_page_id)
-                return queryset.descendant_of(ancestor_page)
             except Page.DoesNotExist:
                 raise BadRequestError("ancestor page doesn't exist")
 
+            queryset = queryset.descendant_of(parent_page)
+
         return queryset
+
+
+class RestrictedDescendantOfFilter(DescendantOfFilter):
+    """
+    A restricted version of DecendantOfFilter that only allows pages in the current
+    site to be specified.
+    """
+    def get_page_by_id(self, request, page_id):
+        site_pages = pages_for_site(request.site)
+        return site_pages.get(id=page_id)