Selling biscuits can get a bit repetitive when selling for more than one week. Add in a scavenger hunt/wide game to keep the girls entertained.
Biscuits to sell
Float for change
Camera per group
List of things to find.
House with a dog (Skip the house)
Photo of girls selling (or with biscuits, get creative)
Collect a piece of rubbish each
Sell all of one box (type) of biscuits
Sell to someone walking along the street
Sell to someone living in a house with a red roof
See a bird