Duolingo helps kids practice and improve the following skills:

Focus: Getting started and then maintaining attention and effort to tasks. 

Duolingo provides the perfect opportunity to practice long-term focus. While it does an excellent job of making the user feel as if they are learning right from the start, it is still only a start. To truly learn the language will take a long-term commitment of time and focus. Learning a new language is not a goal that can be achieved overnight. If the user does not stay focused on their goal, they will learn a few words, but not a new language. For users who struggle with focus, setting the practice time to just 15 minutes a day and then building up to longer practice sessions is advisable. And if the user really needs to take a break, streak freezes are purchasable with gems from the store, meaning you can take a break for a day and come back to the app with your streak maintained. 

Working Memory: Recalling and retaining information in our minds while working.

This app also depends a large amount on using and improving the working memory skill. Duolingo is, after all, all about helping the user practice remembering a whole new language. This requires the user to retain information not only through the entire lesson, but in later days when it is once again being practiced. If they do not use--and strengthen--their working memory, the user will fail to learn whatever new language they are working on. Practicing working memory by associating new words with how they are spelled and then attaching a visual image to the word is a great way to train your working memory. For users who struggle with this, setting your daily practice time to a low level and then practicing the same lesson several times can be a way to feel more comfortable with this process.