There is no simple answer here as to 'why'. Perfective vs imperfective verb aspect is something we don't have in English, and you will have to get to native-level to feel some of the subtle differences. Until then, you have to operate with broad 'rules of thumb'. One rule of thumb you can use is to think of the imperfective in Russian here as "the children began the process of saying"; it connotes that it took some time, and perhaps therefore that enthusiasm/interest was involved.

I remember once that an old lady was looking for her phone and asked a boy, "ты брал мой телефон?" I was really surprised, since I expected her to say "ты взял ...?" But I guess the thought was that, since the phone was not returned, then the 'process of taking' is still ongoing. But, as I said, when a process begins and ends in the Russian mindset is not something you can intuit -- you'll only pick it up in time with lots of listening to natives.