I'm guessing that, if a regular customer who came to buy sweets week in, week out, turned up at Hallowe'en, OP might be more amenable to giving them a couple of free ones, making it very clear to the child that it's 'just because you're a very good customer and it's Hallowe'en!" (could still backfire if they assume it extends to all of their friends and siblings too); but pound to a penny, the kind of parent who turns up doing this is also the kind of person who wouldn't dream of ever paying 'those rip-off prices' - like most people who don't appreciate the difference in economics between multi-billion national supermarket chains and tiny local family shops; nor indeed the huge difference in quality between what the little independent specialist shops sell and bags of Haribo.
I suppose you could always put up a sign saying "Psst! Parents! Ask us in advance about our pre-paid Trick or Treat Evening for your little ones!" Make it clear that the kids can be given sweets, with the parents' agreement, BUT of course, only the ones that the parent has already paid for. You know, just like kids don't pay to go to soft play and, to them, it's 'free'... because their parents have already paid for them.
That way, it gives you an easy 'out' to take the wind out of the sails of the CF parents and show them that they're basically asking for a standard, planned shop 'product', but weirdly expecting not to pay for it for some reason. Also, you may get non-CF parents who spot it and take you up on it, meaning extra trade as their little ones pour in and excitedly leave with a bag of sweets, without ever needing to know that it was already paid for two days previously or see the voucher that their parents discreetly pass to you.
I'd also make it clear that, when customers pay for purchases, it's for products that they have chosen themselves. If they're expecting you to hand over some random sweets, how can you know what the child might not like, be allergic to or contain ingredients that are contrary to their dietary requirements? This kind of non-thinking parent that demand free sweets will very likely be the same kind who would bitterly complain and leave horrendous reviews for your shop if you happened to unwittingly offer them something that their child couldn't eat.