Hi! How do you know which row in the parent repeating table your user wants to add rows to? Is the field with the number inside the repeating table? Or do they click a button that is in the repeating table? Your code is going to get the repeating child inside the first instance of the parent - but if this code runs on a button inside the parent, you can use e.Source to get "this" parent. So - let's say group4 is the parent repeating group and there is a button in there with code for inserting repeating rows into the child repeating group. Let's say the child group is group6.
XPathNavigator test = e.Source.SelectSingleNode("my:group5/my:group6", NamespaceManager);
If inside the clicked event for a button inside group4 would return the first instance of group6 inside the parent where the button was clicked (C# above, but you get the idea).
Hope that helps!