I suggest that the bonus be changed to a universal +2 listen, +2 spot with an additional +2 listen and spot versus halflings, with a total of +4 spot and listen vs halflings. Due to their small stature and racial skill focuses they receive an inherit +6 move silently and +4 hide. Even with this boost to the perk halflings will still have +2 move silently.
I don't think there is any way to implement a dynamic skill bonus against X class or race.
If there is, it's probably more trouble than it's worth. A flat bonus to hide/ms to balance against the small-creature advantage seems reasonable though.
The perk already indicates a plus 2 vs halfling specifically.