If your child says 'I reckon we should leave at six', they are simply saying 'I think' or 'I suppose'. 'Reckon' is everyday English in Britain, Ireland, Australia, New Zealand, and the American South, and it is also creeping into wider youth speech through TV. It is not slang in the disrespectful sense and is not connected to gambling or money in this context. The tone is casual and slightly understated rather than confident, so 'I reckon' often signals a soft opinion rather than a firm claim. Nothing to worry about — it is plain conversational English.