its my first proper contract with people i don't know, so want to make a good impression.

I am holding a place for this LO who is going to be starting in August, so therefore charging a retainer fee,
does £5 a week in advance, so to be paid tonight.. sound ok?
Its only going to be minding for one day a week, so it equates, near enough to a full months money.

I would feel a little cheeky asking for anything more than a months money, but i am holding the place for 5 mths so they should pay something..
what do you all think? am I underselling?