Here is a good article in the New York Times by a professor of psychology who has been working with ADD children since the 70’s summarizing the research that shows Ritalin does not produce long term benefits in children and advocating the controversial position that not all ADHD is necessarily a “brain disease”.

http://www.nytimes.com/2012/01/29/opinion/sunday/childrens-add-drugs-dont-work-long-term.html?_r=1

Here is a link to the Jensen study showing at 3 years children on Ritalin, who had originally done better for the first year, now showed increased symptomology as well as  physical side effects such as short stature and decreased weight.

http://www.madinamerica.com/wp-content/uploads/2011/12/3-year%20followup%20of%20the%20NIMH%20MTA%20Study.PDF (full article)

Here is a link to the original MTA study at 8 year followup showing that children treated with stimulants fared no better 8 years later than those who received no treatment at all.
http://www.ncbi.nlm.nih.gov/pubmed/19318991