I have been contacted by somebody to say Kaddish for their late father who passed away a week ago. There are only daughters in the family no sons. Is it correct to charge a fee for saying Kaddish three times a day for them and if this is indeed so How do I explain it to this family which is secular?

Technically you can charge and explain to them that it is a service like any other service people give. I personally think it will be better not to charge (unless this is your formal profession), you will not become a rich man from the few dollars they will give you, and you can earn a pure mitzvah of Chessed if you do it only for the sake if the mitzvah.