At a sale you can always trace the person who baked it, or at least you can in the US at any sale I have ever participated in. Most counties have a system where the baker is identified by use of stickers or numbers on the packaging of their baked item and in a list held by the organisers. This is so that outbreaks of food borne illness can be traced.
'In 1974, an eight-year-old Houston boy named Timothy O’Bryan died after eating cyanide-laced Pixy Stix while trick-or-treating. Although the poisoning initially looked like it might have been the work of a deranged homeowner, the investigation into O’Bryan’s murder soon centered on his father, Ronald Clark O’Bryan.
A bit of digging revealed that Ronald O’Bryan had recently taken out hefty life insurance policies on both of his children, and police quickly built a case, albeit a circumstantial one, that O’Bryan had given both Timothy and his daughter, Elizabeth, the poison candy to try to collect on the policies. To help cover his tracks, O’Bryan also gave two other children cyanide Pixy Stix. Luckily, his daughter and the other two children had passed up the poisoned powders in favor of other treats.'
It was luck that prevented three additional deaths, one of the daughter of this man and two other children.
mentalfloss.com/article/12914/brief-history-sick-people-tampering-halloween-candy
Again, it's not the fact that this was a family affair that is relevant.
The concern is about copycat incidents that could involve random strangers. This is legitimate, given that some criminals model themselves consciously on others who have committed notorious crimes.