how well did you check the axle bushes? if it only happens with your kids in the back/loaded up they might not be obvious, you ideally want the car up in the air and lever a pry bar in there to check for movement
might sound too obvious but did you check for a cracked or broken spring? i cant see why a dodgy shock would knock unless it was seized or bottoming out, which you'd feel more than hear. When you're done jumping in the boot pop the wheel off and have a look