Index and active object

Hi

I´m a primary teacher and have 12 Android tablets in my class. I have made some small apps (Hrafnkell on Google Play) to use with my class. Now I want to make a small app that helps the kids with their writing. I want them to trace letters.
Like https://play.google.com/store/apps/details?id=net.homeip.alphabet&hl=en
I was thinking of using a background picture of  a letter starting with A. Then I thought I could cover the lines of the letter with small active objects. When then kids move the finger I would change the color of the active object. Am I on the right path here? Now I must ensure that the kids must start at the right place so if the kid would start at the bottom then the colours would not change. If I use duplicate the objects are indexed. Any help. Please excuse my English