Journey to the Windspear Hills - Baldur's Gate 2 Side Quest
The Journey to the Windspear Hills side quest begins by speaking to Lord Jierdan Firkraag inside of the Copper Coronet in the Slums District. He will offer you 10,000G to travel to Windspear Hills and slay some ogres; if you did not already pick up the Helping Irenicus' Captive Dryads side quest from Irenicus' Dungeon then this is the only other way to get Windspear Hills added to your World Map.
Upon your first arrival in Windspear Hills you'll be ambushed by a group of ornery Ogres. No matter what you say to them it will end in a fight after which you'll be approached by Garren Windspear who will tell you that you just murdered a group of paladins. Garren will suggest that you accompany him back to his house as you work out what just happened, accepting his invitation will automatically fast travel you to his house which is a little south of where you started.
If you decide not to travel with Garren right now and leave the zone the only significant change that will happen is at some point in the future you'll be waylaid by another group of Paladins from the Most Noble Order of the Radiant Heart. They'll want revenge for their friends that you killed and will be worth a lot of EXP (but also be a difficult fight).
Back at Garren's house you'll want to talk to him about things that have happened and after some dialogue he will agree to speak to the Most Noble Order of the Radiant Heart on your behalf and basically "call off the dogs". This will prevent the Most Noble Order of the Radiant Heart kill squad from attacking you in the future and also allow you to continue with this quest.
While Garren is gone you'll meet his Son/Daughter (depends on the sex of your main character) and you'll also have to fight off a group of bandits that come to attack. During this bandit attack Garren's Son/Daughter will be kidnapped and this will begin a new quest called Rescue Garren's Child from Firkraag.
Both of these quests have the same goal(s), enter Firkraag's dungeon and save Garren's child. You can find Firkraag's dungeon in the northeast portion of Windspear Hills. The first floor is quite linear and easy but level 2 is going to throw some curve balls at you and spike up in difficulty at parts.
If you need help navigating the dungeon and need tips/strategies on what to do I recommend you check out the Windspear Dungeon Level 2 Overview page that I have. That page will cover most of the hard portions of the dungeon and explain how to get around.
For our quest all we need to do is get to the northern most portion of the dungeon and fight Tazok/Chieftain DigDag. Tazok will drop Sewer Key which is an item you'll need for the Temple District Sewers; more importantly though you'll find Garren's child locked up in this same room.
Now, to free Garren's child what you'll have to do is go downstairs into Firkraag's room and demand the child from him. You can walk away without fighting him quite easily but you will have to fight Conster, a mage, when you return upstairs. He'll drop the key you need to finally free Garren's child.
With Garren's child free all that's left to do is return to Garren Windspear and talk to him to turn in the quest. If you're playing a Paladin then completing this quest will unlock the Paladin Stronghold line of quests.