I wouldn't cut them so much slack. The study is either reliable or it isn't. And it wouldn't take them much more effort to make the results significant in some way.

In this case, I think that just giving these children the self-reported diet would yield massive effects on their health, sugar or no sugar. I am quite sure children with obesity problems don't know their real diet - not because they are dishonest, but because we (as people) are very efficient at hiding the uncomfortable truths from ourselves.