I have a js object loaded on one Onsen UI masterpage, when I called ons.navigator.pushPage('child_page.html') how can I get the js object on the child page?
See the documentation and guides.
http://onsenui.io/guide/overview.html#PageNavigation
You can get objects via the following methods:
page1
myNavigator.pushPage("page2.html", { param1: "value1", param2: "value2" });
page2
var page = myNavigator.getCurrentPage();
console.log(page.options.param1); // Will return "value1"