I can’t help you with the ideas of app creation, so thanks to somebody who answered the question, but I can say that such app is cool in case of learning ABC and shapes, but it won’t help your kid to lean actually write nicely. If you really want him/her to train, print some pages with traces. I’m not the best advisor, because I try to learn writing beautifully, but stumbled upon essayswriting reviews and used the service (no help for training), but it’s a kid! You should understand.