AndrewHi Debbie
We have a 5y.o. salt waster. Sometimes it isn’t a full tantrum but "bad form" or irritability. We reason with him, as in: "...you know you feel tired around this time every afternoon. If you can’t control it, go down to your room to cool off..."
The difficulty is where there are other stressors like sports activity, over-excitement at someone else’s house, hunger, hot weather, not taking on enough liquids through the day and so on. As our GP remarked, at certain stages through the day his levels are off no matter how good the dosage control may be, and this causes him to feel less than great.
Your child is at the age where she is active enough to "run a little ragged" in the afternoon, but still young enough to make it difficult to gain control of it. We have been through this and it gets easier with time and a little effort.
A frozen banana smoothie in the afternoon works for our chap too! You notice the red face or tired look in the eye, and do what you can to head off the actual tantrum before it happens.
Good luck!