I want to make a software that helps learn alphabets by tracing the letters on the tablet/device screen. What software to use to do this?

I strongly believe that connecting dots is a really bad idea (as well as any 3D tricks). This is rather an exercises to be used to train coordination, but for writing letters (and also for teaching drawing) it would be just distracting from more important skills involved in spelling letters, which is different.

I have a different idea: you should show letters as the animated process of drawing them, in the same order and timing (slowed down) as an experience and nice writer does. The student should do the same in a separate area, mimicking this motion. You can also play the audio illustrating each letter. One other aspect of it: it is highly recommended that a beginner got a big area for drawing a letter (and anything else, in fact), as big as possible. It, again, stimulates important freedom of motion and helps a student to avoid distractions for the fine motorics, which should be trained later, when the alphabet is basically mastered.

For technology, any animation technique could be suitable. You can even play video clips, if advanced animation is difficult for you.

Now, my advice is based on my own experience of training children, which is based on some fundamental knowledge but is quite limited. If you can, I would really recommend you to get consulted with a good specialist in pedagogics and, even better, а speech therapist (good specialists also address writing, cognitive development problems and the like). If you have your own ideas, it's better to check them up with such specialists.

[EDIT]

I'm also interested to know what alphabet you want to target, Latin, or some other? I believe, some alphabets are considerably harder to learn.

—SA