Dian, there are many other things that could be a helpful part of your child's anti-anxiety protocol:

Magnesium especially the glycinate form. Check out the US RDA of Mg. So many of us are deficient.

Glutamine or GABA. The latter is the most important anti-anxiety neurotransmitter.

Lithium supplements, maybe just 1 milligram of Li from Li orotate or similar.
http://www.nytimes.com/2014/09/14/opinion/sunday/should-we-all-take-a-bit-of-lithium.html?_r=0

food sensitivities , including foods and drinks high in phenols. The child may benefit from support for sulfate that is involved in breakdown of phenols, natural adrenaline, etc.

Epsom Salt is magnesium sulfate.

Adrenal *cortex* support such as cortex glandular.

Heavy metal detox.
Autism Research Institute treatment parent ratings 2009
http://www.autism.com/index.php/treatment_ratings_asd
"Detox. (chelation)" 74% report "got better", 3% report "got worse", 23% report "no effect"
The 74% rating is the best of the many dozens of listed interventions.