Use the client-side API to rebind your grid, should be the correct way to do that:
In your child page:
function refreshGrid() {
$find("<%= YourGrid.ClientID %>").get_masterTableView().rebind();
}
To call a javascript function from the parent page, simply use this:
function CloseAndRebind() {
GetRadWindow().get_contentFrame().contentWindow.refreshGrid(); // Call the function in parent page
GetRadWindow().close(); // Close the window
}