The number of sit-ups that a 12-year old should do each day is indefinite. According to Exrx.net, the average 12-year old boy and girl can perform 39 and 36 sit-ups, respectively, in 60 seconds.
According to Kidshealth.org, kids as young as 7 or 8 can perform sit-ups as long as they show interest, perform the exercise safely and follow instructions. To perform sit-ups safely, the child should lie on her back with her hands placed on the back of her neck and her fingers clasped. She should bend her knees at less than a 90-degree angle with her feet on the floor and her heels no more than 12 inches from her buttocks. She should curl her torso until her elbows contact her knees and then return to the starting position.