Greenhill L1, Kollins S, Abikoff H,…Cooper T. J Am Acad Child Adolesc Psychiatry. 2006 Nov;45(11):1284-93

This paper describes one of the key studies of ADHD treatment in preschool children.  It broadly showed that stimulant medication is effective for treating the condition, but that lower doses are needed than for older children.

http://www.ncbi.nlm.nih.gov/pubmed?cmd=Retrieve&dopt=AbstractPlus&list_uids=17023867&query_hl=1&itool=pubmed_docsum