I think it’s ok to keep a couple of prizes aside, and add them in once the prizes had started to dwindle a bit - putting in the winning tickets and new prizes later in the day.
Fine as long as the prizes are hidden until their tickets go in (although why would you do that - unless your table wasn't big enough?!); but the basic principle of a tombola is that, if you buy at least one ticket, you have a chance of winning any of the prizes on display.
It's not good enough to try to rationalise it that there are still some prizes in there - firstly, you could have 100 Mars bars and 1 Faberge egg as prizes (unlikely, granted!), so realistically only one actual 'proper' desirable prize; but also, you can't objectively know which prize any one punter would most want, and which may have been their sole purpose for entering.
You also can't try to make people guilty for wanting a fair chance because it's for charity - otherwise you should have just solicited straight donations without offering the lure of a prize to increase participation. A chunk of the income from Lotto goes towards charity/good causes, but can you imagine them getting away with if it were discovered that it was completely impossible for somebody/anybody with a valid ticket to win?
In this scenario, the woman running it should have handed over at least one of the purple prizes, if not both - yes, OP had technically not won them, but it would have been the most fair way of making good the mistake/accidental fraud. I wonder if those prizes would have been carried over to the next tombola (and still not won?) or if, more likely, they were carried to the boot of her own car.