Link to child records not a table of a parent

It sounds like your need then is to create a link that opens up your child records - not specifically with the summary field itself. Summary fields will only open up one level deep in the relationship as far as the drilldown goes. Your comment says that the # is correct so you have a couple options: 

1. Create a direct relationship between your parent and grandchild and create the summary fields accordingly
2. Update the drilldown for your summary field that shows another report of your child records and have on of the columns be the # of grandchildren records. So if the total # of grandchildren is 9 among 2 children records, your drilldown might look like: 

CHILD    GRANDCHILD COUNT DRILL

1                 4

2                  5

     3. Create a formula rich text as custom drilldown with something like: 

var text query = "{'fid'.EX.'" & [Record ID#] & "'}";

//where fid would be replaced with a the fid of a lookup field into your grandchild record for related parent 

"<a href='" & URLRoot() & "db/" & grandchild_dbid & "?a=q&query=" & $query & "'>" & [Grandchild Count Field] & "</a>