I work in an elementary school with kids with peanut allergies. The tables need to be cleaned carefully before the allergy kids sit there, but as long as peanut butter isn't being eaten next to the child, he or she should be safe. It's contact that causes the problem. We haven't had any problems in the past 10 years and we have multiple peanut allergy kids. (Actually, the only reaction we had was a child who ate something he brought from home!)
If my child or myself had a food allergy, I would be wiping down the tray and armrests before sitting on the plane, who knows what was eaten on the last flight.
If I was asked not to eat it, I'd wait.