Instead of adding a row inside the group, try this:

Start with a Table with two columns, one for your "group" value and one for the aggregate (sum):

Select both cells on the second row and Merge them (optional - just for formatting so the subreport can span the width of the table):

Add a Rectangle to the bottom cell:

Add a textbox for the group value and a textbox for the aggregate, then add the subreport:

Now, your subreport will stay on the same page as it's owner.

I guess there are other ways, but this definitely works and is quite simple.