Some of the trials are extremely small. Therefore, sample standard deviation (supposed to be an unbiased estimator of population stddev) may be unusually small or large, giving a very uncertain ES. 12 of the 15 studies included P ratings; 6 included T; 3 included H (Parent, Teacher, Health prof.). I didn't see how the pooled ES estimates were generated (probably by pooling the populations and ratings), but I'm very interested in the ES of different raters on the same population (so that we moot any question about what the true pop. stddev is).

However, Fig. 2 on p 427 of http://www.cspinet.org/new/pdf/schab.pdf definitely shows that parents' ratings get a higher ES on those studies where there are other ratings (on the same population) to compare against. Since all the studies are blinded so that the raters don't know if the kid got a placebo or not, this does effectively prove your interpretation (that either the hyperactivity occurs mostly at home, or that, more likely, the parents can judge a behavioral change in their kid better than a trained stranger who gets a few minutes of observation only).