Here is a recent study (link to abstract below) looking at the use of modafinil, a wake-promoting drug, in children with excessive daytime sleepiness (in this study, primarily narcolepsy).
The study was performed at the University of Louisville School of Medicine, and uses doses of 200-600mg/day -(which is higher than what has been anecdotally reported to be effective for children with PWS). Note that there was exacerbation of psychotic symptoms in one child who had a history of hallucinations -- that has been seen before at high doses in those with prior conditions. Overall, modafinil was found to be well tolerated and helpful. This study is consistent with others that continue to show a generally good safety profile.
Ivanenko A, Tauman R, Gozal D. Modafinil in the treatment of excessive daytime sleepiness in children. Sleep Med. 4: 579-82, 2003.