Well, it’s just one day, and as long as they realise they’re getting it as a treat and that it’s not the norm you should be OK for a while. Maybe keep an eye out for when the bags are being dished out and quickly say “let me look after that for you” then scoop out the really bad stuff when they aren’t looking.
We’re deep into birthday season here - we had one last weekend and there are another four (including Jimbo’s) over the next three weekends.