The most common side effects of Ritalin overall are nervousness and insomnia. In children, trouble with appetite and stomachaches can be even more common. If the stomachaches continue, you will want to report them to your doctor.

If Ritalin is the cause you might want to consider adjusting the dose or changing medications--though sometimes it can be continued as-is. I'm glad you asked this question--many parents don't know that stomachaches can be a common side effect of Ritalin.