All of the Builder's Children are notable for being incredibly resistant to most damage, including initial immunity to Broadhead Arrows and sword strikes, but each possesses a vent on their back which leads to a furnace, their power source. It is this furnace that is their Achilles' heel: 2 Water Arrows fired into it will cause a large Child to shut down immediately (and relatively quietly. A single arrow is sufficient to shut down the smaller ones, either the Worker Bots or the Spider Bots found in Soulforge Cathedral.
All of the Builder's Children can also be shut down by 9 strikes from the Blackjack, though this is both loud and frequently dangerous. A Gas Arrow fired into or near the furnace can also shut them down, but this can be wasteful of resources. Likewise for a few Fire Arrows.
After a Child is shut down, it stands immobile and serves as a corpse that cannot be moved. 8 normal sword strikes will break the body apart into a large core section which can be carried and is capable of breaking glass if thrown, and various small pieces that remain behind for a short while afterwards, similar to blood from living opponents, save that it cannot be washed away. It also bears mentioning that breaking apart the body in this fashion is also rather loud, and may attract unwanted attention.