You will recieve this quest from a spouse when you visit them sometime after returning from the Spire. After accepting the quest, you will be informed that your child has entered the Hobbe Cave in Rookridge. Head to Rookridge and make your way to the Hobbe Cave.
Enter the cave and continue through the tunnels until you enter a circular room with a large number of Hobbes. Dispatch them and continue further. Continue through the next set of tunnels to another open room with a few Hobbes. Make your way up to the top of the room via the ramps in and into the tunnel at the top.
After another brief Hobbe encounter, continue through the tunnel until you see an opening on the right and when you do head through the door. Clear out the Hobbes in here, ascend the ramp and then make your way into the first entrance on your left. Follow the corridor until you reach a large room with a number of enemies and a few cages.
Whilst you are killing off the enemies here one of the Hobbes should drop a cage key. As such, once you have emptied the room of hostiles grab the key, unlock the cage and you’ll be reunited with your child.
Now we just need to escape! Make your way back to the previous tunnel and turn right. Fight your way upwards through the Hobbes and use the ladder to complete the quest.