which benzodiazepine is best to treat anxiety

My psyichiatrist has prescribed xanax for me in the past, and 2 months ago switched me to Valium at 5 mg twice a day for anxiety.  He has also prescribed Cymbalta for me at 60 mg per day for the past 3 years (until 1 month agao, I had been taking 120 mg per day of Cymbalta); and last week he prescibed 10 mg of Lexapro per day to help my depression.  I think that the Xanax helped more than the Valium.  I think that maybe the Xanax worked better.  Is there any evidence that Xanax works better for anxiety.   I am worried a lot about losing my job because of poor performance, and am in trouble with the truancy court because my youngest son, 12 years old, has had excessive absences from school due to poor sleep, headaches, and stomache aches.  He says he hats his school.  Can a child of 12 be prescribed anti-anxiety medication and/or sleep medication?  My anxiety includes excessive worry, fear, PTSD, and stress from a difficult marriage.