Adult supervision required.
Preheat oven to 400 degrees F. Slit each banana skin on the inside curve with a sharp knife (adult supervision please) Do not cut to the banana. Use your hands to loosen the skin slightly, but keep ends together.
For each banana, tuck 1 tbsp chocolate chips and some of the marshmallows along the sides of the banana. Push skin together, it won't close tight. Wrap banana in aluminum foil. Place wrapped bananas on cookie sheet.
Bake in oven for 10 minutes. Remove and let cool for a few minutes.
Be careful opening foil - watch for escaping steam. Eat right out of the skins with a spoon!