I need some advice about my ex childminder?

get some legal advice, and dont respond to any of the letters/calls. it sounds like a catch 22, she has no proof that you didnt pay, you have no proof that you paid. If she was looking after other kids at the time, and was not giving out reciepts to them either, then that could be used in court. Its not your fault she isnt running her business better. goodluck