I'm not a parent, so also fascinated.
Where do we get our sense of disgust about food that has been on someone else's plate?
The fork/spoon goes into their mouth and back down to the plate so the remaining food may have saliva traces on it.
If the guest was an adult, they'd be terribly offended by my expressing a desire that my child is not put in contact with their 'germs' (the child won't understand the fuss, until repetition leads to questioning). Even at school I offended someone by wiping the top of a pop bottle which had visited five mouths, before taking my turn. I genuinely didn't think what message that puts out.
If the house guest was another child, of similar age then they'd also not understand warnings about hygeine and likely not care about my kid pilfering from their plate.
But… I cannot administer corrective instruction until after the offence has played out.
For the marbles, I might try reminding my kid of the last time they hurt a knee or an elbow (bones close to the skin) and ask them to say how tgat felt, then explain how that is how marbles on the forehead feel to their playpal.
Again, I do not know at what age children develop empathy.