Compulsions become negative when they 'take over' and interfere with the person's life - for example if they have to do XYor Z a set number of times before they are able to move on to another task.
Obsessions Unless they are antisocial obsessions can be positive allowing the child/adult to escape into their own world when they find things overwhelming.
You can also use them:
* To share and connect with the child/adult
* As a reward/motivation.
* To encourage the person to learn more about that particular subject; thereby broadening his/her interest and developing knowlegde.