As others have said, virtually any useful app you can think off for iPad there will be android app that is similar. For kids, there is tons of stuff on android for preschoolers, key stage 1 and 2. Help with reading, writing, maths, science, just about every subject you can think off.
Right now my favourite tablet is the Asus transformer which is a cross between a netbook and a tablet.
At the cheaper end, Advent Vega will do pretty much everything you are looking at doing and can be found on ebay for around £170 or in shops for around £200.