This happens during Troll Trouble and is probably timed. You can also fail this quest if the child dies. You'll need a caster with glitterdust prepared. Linzi can learn the spell while leveling up, or Octavia can learn it either leveling up or with a scroll. A mother will approach you and ask you to find her son. Agree. Go to Swamp Witch's Hut. Talk peacefully with Old Beldame. Return to your capitol and talk to the mother again.
Go to Lizardfolk village, southwest of the capitol. Use anything that will buff your Knowledge arcane and persuasion. Heroism is very useful here. Save. Again, approach peacefully. Speak to the chieftain and convince him to part with the child (pay him or persuade him). Talk to the Lizardfolk around the village. To the south, two are arguing about farming, tell them to think for themselves. To the west, ask Fashor wants wrong, and tell him you just want the child. North, in the cultch hut, heal the lizardfolk.
Now, go rescue the child. If you pass the Knowledge Arcana check and have glitterdust, you'll have an easy fight with a will-o-the-wisp. If you fail the skill check, don't fight. Reload or give up Kagar to the spirit. This will free Tig. Return to the capitol and talk to the mother.