Advice needed please.

I have a lady coming to see me regarding having her little boy, starting in Feb. If she decides to sign up, should I charge a retainer fee and if so, how much do other people charge? I was thinking of saying bring your little boy one day a week until feb (then the days she wants as from feb) so I i'll only receive payment for the time I have him, rather than a retainer fee. What do others think?