Add serializer for group children

app/controllers/groups_controller.rb

@@ -60,7 +60,22 @@ class GroupsController < Groups::ApplicationController
     end
   end
 
+  def children
+    parent = Group.find_by(parent_id: params[:parent_id]) || @group
+    if parent.nil? || !can?(current_user, :read_group, parent)
+      render_404
+    end
+
+    @children = GroupChildrenFinder.new(current_user, parent_group: parent, params: params).execute
 
+    respond_to do |format|
+      format.json do
+        render json: GroupChildrenSerializer
+                 .new(current_user: current_user)
+                 .with_pagination(request, response)
+                 .represent(@children)
+      end
+    end
   end
 
   def activity

app/serializers/group_child_entity.rb

+class GroupChildEntity < Grape::Entity
+  include ActionView::Helpers::NumberHelper
+  include RequestAwareEntity
+
+  expose :id, :name, :description, :visibility, :full_name, :full_path, :web_url,
+         :created_at, :updated_at, :star_count, :can_edit, :type, :parent_id,
+         :children_count, :leave_path, :edit_path, :number_projects_with_delimiter,
+         :number_users_with_delimiter, :permissions, :star_count
+
+  def type
+    object.class.name.downcase
+  end
+
+  def can_edit
+    return false unless request.respond_to?(:current_user)
+
+    can?(request.current_user, "edit_{type}", object)
+  end
+  expose
+end

app/serializers/group_child_serializer.rb

+class GroupChildSerializer < BaseSerializer
+  include WithPagination
+
+  entity GroupChildEntity
+end

spec/serializers/group_child_entity_spec.rb

+require 'spec_helper'
+
+describe GroupChildEntity do
+  let(:request) { double('request') }
+  let(:entity) { described_class.new(object, request: request) }
+  subject(:json) { entity.as_json }
+
+  describe 'for a project' do
+    let(:object) { build_stubbed(:project) }
+
+    it 'has the correct type' do
+      expect(json[:type]).to eq('project')
+    end
+  end
+end