Cecilia and Cornell have had enough. Instead of living the life of normal children their mother has forced them into a regimented existence that has stifled their creativity and freedom. Sure they know more skills in their early youth than most Sims would know in a lifetime, but at what cost? They've run away to live with their grandparents hoping to find out who they are and who they want to be before adulthood sets in.
Cornell and Cecelia's mother is neither shown or seen in-game, but it's possible that their grandparents are maternal since they have a different last name. It could be an allusion to Severus Snape (his mothers maiden name was PRINCE) and Lily EVANS from Harry Potter. They were very close as children.