its not just about kashros, its about their 5 year old with a runny nose helping and licking the batter.
Actually, the OP made it about BOTH cleanliness and kashrus.
And not everyone has little children and even those that do don’t allow them to put their runny noses in other people’s (potential) food. In my household, for example, when my kids were small, they could have leftover batter — only after the cake itself was in the oven. I would *never* allow them to stick their fingers (clean or otherwise) into the batter before it was poured into the pan.
The Wolf