Opening a bag of nuts is very different than eating a peanut butter sandwich. We have a very good friend that has so many food allergies, it would curl your hair....it's a huge deal traveling with her. BUT...she has told us that eating a peanut butter sandwich next to her isn't going to be bad, but having bags of nuts can be. Others may have different needs.
But again, I ask....what kind of risk is a parent taking by taking a severely allergic child on a plane that has not been cleaned properly? It's a much bigger risk to chance sitting in a seat/area that has peanut oil on it! Yes, you can try cleaning it, but man, that's going to take a long time. If it were my child? That child would have gloves on (latex probably) and a face mask. I would not take the chance of my child touching anything!!!
I just wouldn't rely on the 'goodness' of other passengers. Do I care about other kids? Of course I do. But parents have to take responsibility for their kids' welfare....trusting complete strangers to comply just wouldn't get it done for me. I can just imagine the guy two rows back who feels that he is far enough away and eats his peanuts, after shelling them. Talk about a risk!!!!