While I myself am not a big fan of kobolds as being player characters, there is one person who plays in my campiagn that is. So, I (with input) decided to make the kobold ability score adjustments as follows (and keeping the rest of the Pathfinder Bestiary stats):
-4 Strength
+2 Dexterity
+2 Constitution
I never really went along with the idea that kobolds weren't tough. In fact, I have always envisioned the opposite. While they are smaller than gnomes, goblins, and halflings (as how we play them in my campaign), they are still reptilian humanoids, and thus I have always seen them as being tougher than how they are represented in most of the related material I have read.
On a side note, I almost considered lowering their Strength to just -2, as per the other Small races, and reducing their speed to 20 feet as a balance, but ultimately decided not to since my vision of kobolds has them being on the low end of the Small size category (slightly taller than Tiny size) -- thus, the reason for their -4 Strength.