What's the prevalence of interpersonal psychotherapy and cognitive behavior in the treatment of depression? Is there any link, e-book, whitepaper that you can provide me to help me determine the prevalence of IPT and CBT amongst kids 7 - 12 years and adolescents 12 - 17 years who suffer from Major Depressive Disorder (MDD). This is for a clinical trial campaign we are doing due diligence for. I need to know if we can target enough number of participants to start a study.
The mental health foundation (MHF) UK do some good work here, collecting data about prevalence of mental health problems and engagement with therapy. I have linked to a 2015 report which summarises a plethora of research on the depression, including MDD. You can follow the references to find information about the prevalence of MDD and engagement in therapy. For example, the paper references a 2006 meta-analysis of the prevalence of depression between 1965-1996 (USA statistics)..."overall prevalence estimates were: under 13, 2.8% (standard error (SE) .5%); 13–18 5.6% (SE .3%); 13–18 girls: 5.9% (SE .3%); 13–18 boys: 4.6% (SE .3%)" (Costello et al., 2006). A UK government report basically puts this prevalence at similar levels to the US. Going back to the USA, a 2014 report puts the incidence of depressive episode experiences at 11% of the US adolescent population. All I can say is that I'm glad I'm not a teenager anymore!   

Therapy engagement and age is a bit more difficult to obtain, especially for interpersonal therapy in younger age groups. In the UK 38% of all therapy in primary healthcare (basically non-hospitalised patients) was CBT in 2013/14. In the annual NHS report for Improving Access to Psychological Therapies 2014/15(IAPT) puts this at 34% (1.2 million) by contrast interpersonal therapy was only around was just over 1% (40,000) of appointments. Under 18s age groups are not given, but it is reasonable to assume given the kind of therapy available for adults in the UK the rates would be similar for children, particularly because CBT training is more readily therapy. If your in the US it may be important to consider that therapy engagement maybe lower in younger age groups. 

TANGENT
Worryingly and personally the most horrible statistic I have read all week is that depression in children is a risk factor to mortality comparable to smoking. However meta analysis have reported that CBT is 51-87% effective in treating depression. A Canadian report puts psychotherapy (CBT and IPT included) as .66 (cohen's D) more effective than no treatment at all in children with depression. The report does not compare success rates of either therapeutic technique. However a study has been planned to compare therapeutic treatments in children and adolescence, it has not been published yet. Rates of recovery in adults (which is often lower than children) may be an indicator, with IAPT reporting 58.4% recovery with CBT and 53.9% with IPT (remember the small sample size for IPT in IAPT). Within anxiety research cognitive therapy is found to be far more effective than IPT. One study appears to suggest that CBT is also more affective with depression than IPT, although a slightly older study found no significant difference, which looking at the IAPT data would seem to be more likely.  

I hope that is helpful, I would also recommend looking at the USA based Nation Institute of Mental Health website, they have a lot of information and links to research.