Not a misheard, but still funny, I think. Older grandson aged five and his friends were trying to persuade younger grandson, four in a few days, to play monsters. YG had obviously taken on board that he was going to be a big boy, pulled himself up to his full height " I'm grown up, I'm too old to play stupid games".
OG patting his tummy "Look at my sixpack" YG patting his chest "Look at my backpack".
Ds