Hello A.,
My 16yo daughter has severe ADHD,and I also was a teacher for 17 years, so I have quite a lot of experience with this...even though I am not a doctor. Yes, caffiene has a calming effect on those with true ADHD. Prescription ADHD medication are actually stimulants, as is caffiene. Stimulants have an opposite effect on those with ADHD.
Also, a funny little story I'll throw in. Whenever we would take kids on a field trip, we would always stop at a place where the kids could buy sodas for the trip home after the field trip. (I was a junior high and high school teacher). The students thought they were getting a wonderful treat, but the teachers were doing it so that the kids would calm down for the ride back to school!!!
I have allowed my daughter to drink coffee since she was 3 years old, and I have allowed soda on occasion. When my daughter was young, I would give her about 1/2 cup of coffee and 1/2 cup of milk, and put sugar in it so she would drink it. It helped her concentrate on her school work.
We tried 3 different ADHD medications and with all three, the side effects weren't worth the benefits she recieved. So, caffiene is our ADHD medication of choice.
Okay, I know it doesn't sound like the healthiest alternative, but I did ask our pediatrician about it, and she said, and I quote, "She can have all the caffiene she wants."
Oh, and my stepson (to be) has ADHD and Asperger's Syndrome, and when on a long trip to upstate New York, we gave him Monster Energy Drink, and he went right to sleep! He is 14. I wouldn't recommend this for a 5 year old, however.
Hope this helps,
L.