If I recall correctly a cubic foot of helium can only lift about an ounce of weight. So to lift 50 pounds you would need 800 cubic feet of helium. If the balloon was a 20 foot diameter circle, it would only have to be an average of 2 1/2 feet thick to hold 800 cubic feet of helium. It was fatter than that. I assume the balloon itself was made of very light materials. Of course you have to correct for the altitude of Denver, but basically it looks like the balloon could have lifted the kid.
I should have listened harder when that professor from Pakistan was trying to teach me physics back in college.