When do you allow your pre schooler to experience junk food?

It is okay for preschoolers to have junk food once in a while as their body should get used to eating junk as well. Kids have very sensitive stomachs and they get adapted to the food habits that they follow. So if your child is only eating healthy food all the time, he would fall sick the moment he eats junk food. It is always better to maintain a balance. While am not saying that you should be giving junk food to your child every day, you can definitely give him a burger or a pizza say once in a month or two weeks. It's totally fine. Didn't we have them when we were kids?