How to add or access related object or related list record details in Visualforce email template in Salesforce

          Visualforce Email Template supports to access a related list or related object records by using relatedTo.reationshipObjectName.

In the below example, showed Contact and Opportunity records related the Account object.

Here,

Account --> Parent Object

Contact and Opportunity --> Child Objects

In email template,

Contact records accessed by relatedTo.Contacts

Opportunity records accessed by relatedTo.Opportunities

Sample VF Template:

1. <messaging:emailTemplate subject="Your Detailed Contact and Opportunities" recipientType="User" relatedToType="Account">
2.     <messaging:htmlEmailBody >
3.         <p> Dear {!relatedTo.Name}, </p>
4.         The following customers are waiting for your approval,
5.        <ul>
6.             <apex:repeat var="cn" value="{!relatedTo.contacts}">
7.                 <li>
8.                      {!cn.Name}
9.                 </li>
10.             </apex:repeat>
11.        </ul>
12.        The following opportunities are ready to close,
13.        <ul>
14.             <apex:repeat var="cn" value="{!relatedTo.opportunities}">
15.                 <li>
16.                      {!cn.Name}
17.                 </li>
18.             </apex:repeat>
19.        </ul>
20.         <p> Thanks, </p>
21.         <p> Sales Team </p>
22.     </messaging:htmlEmailBody>
23. </messaging:emailTemplate>