How much methylphenidate or dexamphetamine will a child with ADHD normally need to take?  

Individual. Adhd: clinical practice guidelines for the diagnosis, evaluation and treatment of children and adolescents with attention-deficit/hyperactivity disorder were updated on october 16, 2011 to specify that the dose of medication must be titrated to achieve normality in every individual, adverse effects being the only limiting factor.
Not predictable. Every child/person is different. I always "start low & go slow" titrating the dose until it is a fit. My experience is that doses have almost nothing to do with age, weight, or even severity of add. Rather they are treating that person's unique neuro-chemistry and the person's specific challenges & situations. Some very small, very hyperactive kids metabolize meds very quickly, needing huge doses.
Variable. The proper dose of add/adhd medications is completely unique for each child. There is no accurate standard dose that fits all and unfortunately much trial and error is often necessary for proper adjustment due to each child's different metabolism.

Related Questions

How often does a child with ADHD need to take methylphenidate or dexamphetamine?  

Daily. Adhd is a neurobiological disorder that requires daily treatment. On weekends, on holidays, all the time. It's not just for school - it's for everything. Depending on the formulation of methylphenidate or dextroamphetamine, doses may be needed every 4 hrs to once daily. Read more...
It Depends. There are many differents types of medications with different modes of action, length of effective duration and method of administration. In addition, each child metabolizes the medication uniquely. Sometimes 1, 2 or 3 times per day depending upon which medication you are using . You need to discuss this with your doctor. Read more...
Depends. This varies with the drugs duration of effect & treatment philosophy. Some kids need it year round because of their degree of impairment. Some can just use it for school days & be off on weekends/summer. The primary goal is good function when focus is needed while allowing the kid time off to be themselves when appropriate. Short acting meds may be dosed every 4 hours while sustained release 1xd. Read more...