Estimates of the average number of families benefiting from the child care element of working tax credit are provided in table 2.4 of the HMRC publications “Child and Working Tax Credits Statistics. Finalised annual awards”, for each relevant year. These are available on the HMRC website at:
http://www.hmrc.gov.uk/stats/personaltax-credits/cwtc-quarterly-stats.htm
Information on the total number of families eligible for the child care element is not available.