Any chance you can make them Saturday? Otherwise make them, freeze them and defrost Saturday night. They'll dry out if you make them that early.

I wouldnt ice them until Sunday morning. Buttercream needs to be refrigerated, which dries out the cakes. You could make the icing Saturday night, keep in the fridge and ice in the morning. You just need to soften it a bit first.

It's usually adults buying cake things at fetes so passionfruit should be fine. Otherwise plain vanilla with coloured icing and sprinkles or smarties work well.

ETA - just saw posts above. Anzac biscuits store well. Or honey joys.