What greed? The children are being taught to help others by collecting tzedakah for others, they aren’t collecting for themselves. The mitzvah is tremendous.
As far as during davening, when else can they reach out to so many people? They are collecting for desperate, less fortunate people. Besides the mitzvah being tremendous, lets not forget we would much rather be the one facing an outstretched hand, than being the one stretching out our hand.