Use Case:
Is it possible to merge information from two different child objects, starting from one of the child objects? For example, can I select a child object in template builder and then another child object and then descend into another related list?

App Expert Reply:
You can only merge related list from one child object level deep in template builder. If you want to merge related list from another child object, you can relate that child object to the parent object being used as base object in document action. Then you can merge from that object as the direct child and then from that related list.

If creating the relationship to parent object is not a preferred option,  in order to sum child records in Salesforce, you need to use Rollup summary field for Master-Detail relationship Objects. If you want to sum up child records related through Lookup relationship, then the easiest way would be to use Lookup Rollup Summary 3rd party App: https://github.com/afawcett/declarative-lookup-rollup-summaries. FYI this Lookup Rollup Summary app will not work if using Professional Edition. Another option for rollup summary app if using PE is “Rollup Helper”: https://appexchange.salesforce.com/listingDetail?listingId=a0N30000009i3UpEAI.