"You're breaking my heart." It was "somewhat" cute the first time, but manipulative just the same, and then her brother decided to try it out too. Children sure do have a vast array of tactics to play on the heartstrings of the adults in their lives. Just try to discipline a rambunctious youngster and he or she is sure to come up with a creative attempt to coerce you otherwise. Tearful pleas, negotiating, and angry threats. I've heard them all.
I learned much in the raising of my own children. I made plenty of mistakes, some of them out of sheer frustration, others out of complete exhaustion. I've found a simple "time out" chair to be quite effective (it doesn't leave welts...) as well as said child being left to cry it out, if my nerves (and those of others present) can handle it. One small child told me not so long ago, "I'll cry." I was unmoved and so he said, "I'll cry and get mad." I assured him the tactic would be ineffective and he didn't bother to try either one.
Disciplining children in crisis is hard. They are tired, they are frustrated and angry, and they are hurting emotionally. On top of that, they are little and naturally naughty. Consistency is of utmost importance and yet so hard to implement. I'm still searching for what works, especially for one soon to be four year old with a desperately hurting heart and bad attitude.