It is not about the salad which is composed of veggies and fruits being unhealthy. These foods, if fresh, are beneficial for the boy. The problem lies with food handling and the risk of salmonella or E.coli. This is the reason why you need to stay away from bagged salad from supermarket shelves.
Bacteria can be found on veggie leaves and it is not right to eat it. It’s all about food contamination – why risk it? It would be better to make your own salad at home, less risk and more nutrition.