I can only highly recommend the DUPLO WORLD app for Android. Our daughter loves to while away the time.
The first level is free and it already offers a lot of fun and learning for the little ones. The other levels can be bought additionally.
The prices seem a bit more expensive; but for such high quality 3D games I think the price is justified. Because I know roughly how much effort it takes to program such a game.
In short: a successful child-friendly game app for children up to 5 years of age.