randy johnston , 68 , from dallas , texas , decided to leave two ` fake feces ' on his granddaughters ' beds .
his son then went about filming the moment of discovery .
but footage shows that the prank turned out to be decidedly disastrous , with randy 's six-year-old granddaughter porter crying in horror .
randy , an attorney , told daily mail online that he staged the prank in revenge for one his granddaughters pulled on him earlier in the day .