There is an amazing new app called StudyLock. It locks kids android devices until they answer questions in Math, Science, History and other subjects. Configure the amount of questions kids have to answer everytime they open their phone and /or apps. It cannot be uninstalled except with a parental password. StudyLock also tracks the android usage, grades the kids on all subjects, and kids compete for prizes on the website. Itis a great educational lock. Try the free version.