Assalamu 'alaikum wa rahmatullah,
If somebody was saying something like this "Children wake up early during Ramadhan because they enjoy the time, then after Ramadhan why don't they wake up for Fajr? What's so fun about Fajr? The children think what's so fun about Fajr, we know the importance of Fajr..." etc does it constitute kufr? The person had no intention of mocking the deen but the words came out and although it seemed to be a representation of what the children would think it sounded like coming from themselves- is this just a doubt? There was no intention to mock the deen of Allah but these words came out.