Prevention: In the moment after DD2 had taken a bite of DD1's toast, and the toast intended for DD2 was still untouched, I would have said, "Whoops! I guess THIS one is yours!" and switched the plates. Unless, that is, DD1 thinks it's very important to have her toast whole instead of cut in quarters.
I find GD works in theory so great. Then you're in a situation where "logical consequenses" lead to even worse logical consequenses -- hungry, tantruming child leads to more tantrums! What's a mama to do?!?! Just realize my home is the real world, not a parenting book. And it's OK to make mistakes so long as apolgies and forgiveness are exchanged later. I'm a black and white thinker, though. Which leads me to want to translate book theory into the real world. There are just too many grey areas in parenting to do that, though. I think one of you mamas already gave me similar advice.
Can I also just say that tantrums suck? Esp now that she's getting older. My tolerance for them is waning. She's mature so much of the time. So when she does have a tantrum I'm caught off guard, and I expect her to behave like someone who is older than she actually is because she so often does!
Y'all rock. My fears of posting here now seem unfounded and have evaporated.