The Kingswood Children are remnants of Harys Baratheon's army, Reach soldiers, and sellswords gone rogue after the outcome of The Battle of the Kingswood. Their name was an attempt to attract young orphan boys, and the leader was a pedophile.
The Children ambushed Lord Bonifer Tarly and his party of retainers with a shower of arrows in the Kingswood. In a matter of minutes, they killed two retainers, injured Tyro and took Beren Merryweather hostage.
Bonifer and Ser Varus attended the court of Queen Danae Targaryen and besought her aid in routing the bandits. She dispatched a group of knights led by James Rivers and Ser Ryman Sunglass to defeat the band of cutthroats and make safe the roads through the Kingswood.