In order to avoid any confusion, I think it is also worth mentioning that if you are using any other services (e.g. $http, $cookies, $state) in your childService, then you also need to explicitly declare them.
e.g.
function() {
var childService = function($http, $cookies, parentService) {
// Methods inherited
this.method1Inherited = parentService.method1();
this.method2Inherited = parentService.method2();
// You can always add more functionality to your child service
angular.module("app").service("childService", ["$http", "$cookies", "parentService", childService]);
}());
You can either declare the services you are using inside your child in an array and then they get injected automatically, or inject them separately with the $inject annotation: