Your lessons will very engaging I believe as preschool children are ICT savvy and they will be engaged in their lessons when mobile applications are used. You will be able to achieve your learning outcomes with ease. However , challenges are inevitable I'm sure. Close monitoring is vital.
But research shows that when applications are used by children for a limited time (10-15 minutes per day) and their parents control them, this is not indicated as a negative influence on the children.
From the development point of view, I would make sure the interface fits both phones and iPads. This will give your app more scalability. Language-wise I would also make it scalable so that you can add translations along the road. Do not hard code it. Good luck!