In my clinical practice, I have seen a number of children and adolescents who suffer from Obsessive Compulsive Disorder (OCD). Here are different examples of how OCD presented in 3 of my clients:
(1) A 15-year old teenage girl reported that she had an uncomfortable feeling inside telling her something was not okay or not quite right. She had developed all kinds or rituals to get rid of that feeling so that she could continue on with her day. 
For example, she had to flush the toilet handle multiple times and repeatedly turn the faucet sink handles, turn the lights on and off repeatedly, check and re-check her hair iron to make sure it was off, do certain things an even number of times only, and perfectly align and order her clothes in her closet and in her drawers.