If none of the "children" is stuck somewhere in a wall, you might need to load a previous save.
I had this same problem as well, and reloading an old save didn't change anything. After much hair-pulling and trial-and-error, I figured out how to move past this particular glitch:
Activate console commands ( http://dragonage.wikia.com/wiki/Console_(Origins) ) and type "runscript zz_vgk_debug". A dialogue menu will pop up. Select the following:
1 - Jump to location
3 - VGK Siege
1 - VGK400 Siege
2 - Plot jump
5 - More
2 - Final battle
Immediately talk to Captain Garevel; the cutscene that SHOULD have triggered after you killed the Children should trigger now. Then kill the new darkspawn, and continue the battle as normal. Things might happen a little out of order, and you might have to run back and forth from the courtyard and front gates a couple of times, but this is the only fix I could find that works. I hope this helps!
Spent hours looking for a solution and this is the only one that worked
