It has been shown that most children get a slight high from excess sugar; some just react more than others. So limiting sugary foods is usually a good idea. Also, many children with hyperactivity (or Attention Deficit Hyperactivity Disorder) are intolerant of food colourings & additives, and/or of other foods.
As a consequence of these reactions they have been demonstrated to lose essential nutrients such as Zinc in the urine, and become deficient, which makes the problem worse. Treatment with diet and supplements can make a major difference. In a minority of cases desensitisation is necessary to control the problem.
Please understand that we dislike putting needles into small children as much as they (and you) do; we prefer to avoid this as much as possible by using other sampling methods such as urine and sweat. Sometimes a needle cannot be avoided, and we will do everything possible to keep it to just the one. For allergies/intolerances, we try to find the answers by diet as much as possible.