I would assume that if the first one eats 1/3 of the cookies and the second one eats "the same amount" it isn't specifically 1/3 of the plate, it is just the same amount of cookies the first one ate. for example, (not the actual answer) if there are 27 cookies, and the first child at 9, and the second one ate 9 too you would have 9, then the 3rd child ate specifically 1/3 of the remaining cookies, that would explain how the math works. then there would be 6 cookies left still, even though by the way to question was worded, it sounds as though each child at 1/3 and it adds up to 3/3's. You just have to look closely and realize it is never specifially said that the second child eats 1/3 of the cookies. just however many the 1st one ate. which is more than 1/3 cookies when there are less cookies. its confusing. I know.
I won't answer the question, ill leave that for you to work out. but hopefully I gave you at least a bit of info to work off of.