What a great way to train fingers for handwriting! Many apps for touch screens encourage us to move around the screen with the index finger.
Sadly, the thumb dominates in much of our electronic world. It may well contribute to some really bad pencil holds. The thumb wraps around fingers and the palm is closed. The pencil is clenched in a fist-like hold.
There are lots of alphabet apps for young children, but choose wisely. If letters or numbers are shown, be sure stroke directions and sequences are included. Most important, they should comply with the characters that are in the program you use for handwriting instruction.
Then there are drawing apps that might help adults as well as children. It will usually come naturally to draw or write with the index finger on a screen, and the hand will probably be relaxed. A child will have fun if you show or demonstrate a letter or word, then let the child write it on a screen.