Their effect size measure is "standardized mean difference" which just means that you subtract one mean from another and standardize your scale by dividing by the standard deviation (although there are a few variations, depending mainly on how you estimate the standard deviation). So, for instance, ES = .441 for parents means something like: parents whose kids received the test substance reported their kids as being about 0.4 standard deviations more hyperactive than parents whose kids received placebo.

This statistic does not have a maximum of 1. The general convention for Cohen's d (a common version of standardized mean difference) is that an effect size of 0.2 is small, 0.5 is medium, and 0.8 is a large effect.