Efficient way of limiting selection to child or parent items

I have two separate scenarios.  In both scenarios the treeview is 2 levels deep.

 Level 1A

     -Level 2a

     -Level 2b

Leve l 1B

I have hacked together some jquery but it is ugly and  I was wondering if there is an efficient way to in one case limit selections to top (parent) level nodes (Level 1A, Level 1B) and in the other case limit selections to child level nodes (Level 2a, Level 2b).  Maybe someway in the template to add a class to child level nodes or something of that sorts?