when kidnapping Kiyi one of Azula's henchwomen notes, "the sedative wore off early on this one" (smoke and Shadow pg 157) This implies Azula and Co. were drugging all the kids, to make the kidnappings go smoothly. The sedative would need to meet a few criteria, as the window of time Azula and Co. had to grab the kids was small, I can't imagine they were waiting for the kids to eat/drink drugged food. And drugging the food/drink beforehand would require another visit which would increase chances of being caught. Also the kids would cry out for help if the sedative was not fast acting.
What I find interesting is Azula and Co. know how to properly dose and administer sedatives. Children have smaller bodies so the dosing of medicine has a much smaller margin for error then adults. These fact does begs some interesting questions. Did the Mental Institute use sedatives? That would explain Azula and Co. skill in dosing sedatives. Maybe the Royal Academy for girls has a great medical training program. thoughts?