There are several baseline apps that have been really useful.
World Facts Free - has the flags and countries of the world, as well as some baseline data about these countries.
History: Maps of World - Gives visual maps of the world and what they have been part of over time. You can choose categories or a specific era.
Geo Walk - walk the globe and find out about famous places, animals, plants, people around the world. Complete the quiz at the end.
Brain Pop - this doesn't give you full access to the brainpop site - instead it has a featured movie of the day with a quiz.
Dinosaurs: The American Museum - has basic facts and information about the different dinosaurs.
Dinosaurs unleashed Free! - has flash cards and a quiz about dinosaurs.
Animal Encyclopedia - basic information about the animal kingdom, groups of animals, fauna, animal names and kinds of animals
Animals 360 - a very basic app suitable for younger children. It incorporates photos and sounds of the animals, a tour of the farm, safari and forest, a quiz and photo puzzle.