Maintaining a proper body posture during interactions with educational tablet applications is crucial for children's physical well-being and task performance, especially considering digital tablet's increasingly pervasive use in classrooms. In this work we propose WriteUpRight, an interaction system for children's self-regulation of posture while writing on a tablet. The system relies on slowly deforming visual stimuli appearing on the tablet screen and compares two posture correction strategies: the Error Amplification method seeks to induce self-correction by amplifying the postural error, while the Error Correction method seeks to unobtrusively nudge the child towards the correct posture. Through a formative design and a user study with 42 children, we demonstrate the effectiveness of our solution and the advantages of the Error Amplification method with respect to the Error Correction method. The system shows potential for helping children maintain a proper head-screen distance and head roll angle during reading and writing tasks on tablets.