Are you mainly thinking of this as a drop-in centre for families on a regular basis while doing the weekly shopping or for an hour or two in the mornings sort of thing?
Whilst it is an interesting idea that could potentially be successful I would have to say no, I don't think I would use a drop-in facility. Personally I would not leave my child in an environment that they that they were unfamiliar with and with people that I did not know. Normally in a creche the child gets an opportunity to go through a period of transition where they make several visits to the setting with their parent(s) to get to know the place and the staff and stay there for short periods of time built up over a couple of weeks. The chances are that this could be difficult in a drop-in setting.
Just a few things to consider in any case - to cover yourself in terms of ratio's you would need a minimum of two staff at all times to ensure the children are constantly supervised even throughout lunch breaks, running to the loo etc. as obviously there must be someone watching them constantly. Just going on the rate of minimum wage (8.65 per hour) you would need to have a minimum of four children per hour, every hour just to meet the cost of staff, before the cost of rent, rates, utilities and bills, insurance, equipment, admin costs and so on. Never mind being a profitable venture. Do you see this as being feasible long term?
Have you got experience in a formal childcare setting and are you familiar with the types of policies and procedures that you would need to have in place?
Have you looked into the necessary space required per child and ratio of staff to children in the pre-school regulations?
Just a couple of thinking points that might help you out if you havn't considered them already.