HMRC do not produce these statistics separately for child tax credit and working tax credit.
Estimates of the number of families with tax credit awards in 2006-07 in each constituency, based on final family circumstances and incomes, are available in the HMRC publication “Child and Working Tax Credits Statistics. Finalised Annual Awards 2006-07. Geographical Analyses”. This publication is available on the HMRC website at:
http://www.hmrc.gov.uk/stats/personal-tax-credits/cwtc-geog-stats.htm
The same information for 2007-08 is not yet available because awards have not yet been finalised.