Your grandparents are the characters that follow the very first Tamagotchi character you see when you first see the 'parent's' section. There won't be a sing saying that they are, etc. but thats who they are.
Are you thinkig of :Tarakotchi? If so- then it is pretty simple to get your Tamagotchi to evolve into him. Just take very poor care of the teen and toddler, feeding it too much snack's, etc.