[SalesForce] Custom Button : Conditional Redirect to URL using Javascript

You can do this a number of ways..

1. The easiest is to use AMartin's hack here: https://success.salesforce.com/questionDetail?qid=a1X30000000KDqCEAW I use the term hack deliberately, because salesforce may not support this in the future.
2. Create a visualforce button that updates a field on the record and fires off a workflow email alert. You can choose the email template that email workflow alert fires off of.

Most browsers have a console where you can view JavaScript errors. Right click on the page and inspect element, then click on the console tab

Personally, I would do this with a VisualForce Page which, being top level, would not have any framing issues and will clearly display any errors:

<apex:page standardController="Case"
    extensions="CreateCaseChildrenExtension"
    action="{!createChildren}">

    <apex:pageMessages />

</apex:page>

And a controller extension:

public with sharing class CreateCaseChildrenExtension()
{
    public PageReference redirect { get; private set; }
    public CreateCaseChildrenExtension(ApexPages.StandardController controller)
    {
        this.redirect = controller.view(); // If you want to redirect to the Case
    }

    public PageReference createChildren()
    {
        try
        {
            // insert children
        }
        catch (DMLException dmx)
        {
            ApexPages.addMessages(dmx);
            return null;
        }
        return redirect;
    }
}