That's up to you, but there is a reward already
Kingmaker is a very sandboxy campaign, and if you actually sum up all the treasure, the party will likely be over their Wealth by Level. Especially if you add some random encounters during their exploration. The reward quest for that is explained on page 34:
Reward: Delivering the statue to Tartuk wins the PCs safe passage out of the kobold lair as well as a shiny 500 gp topaz and (if the PCs ask for it) Svetlana’s wedding ring from the shaman. Delivering the statue to Chief Sootscale wins them instead the ring, the shaman’s gear, the eternal thanks of the kobold chieftain and, perhaps, an actual alliance with the Sootscale tribe.
Keep in mind that the quest given by the swordlords (which is a different quest) is to keep the kobolds activity under control, and resolving their conflict surely is a way of controlling it. That reward should be given regardless of the outcome. But of course, they will likely get a higher treasure total if they get the reward and also murder all the kobolds.
However, if you believe they are under WBL, you may also fix that by adding a few more exploration encounters that will reward them with treasures. But, by no means, the kobolds have to give the party their valuables.
In my group, the kobolds were hired to be scouts and miners in our kingdom, which translated as a free mine once we absorbed their hex and a tiny army of scouts that never actually got used, but were there defending the mine hex, which was far more valuable in the long run to us.