Language learning apps are a lot of fun and for kids Duolingo is really effective and entertaining. As you mention the fact that they are short activities makes it very attractive because children do not get bored. Of course the app drives me crazy when it reminds me to take a few moments to do my homework ha ha ha ha.
RE: Duolingo and learning a foreign language for children/Duolingo y el aprendizaje de una lengua extranjera para niños. [Eng/Esp]