Kids don't care about things like processor speed, memory or screen resolution. They just want something that works.
If money isn't an issue, then I'd go with the iPad mini. The App Store is huge: almost every app comes to iOS first.
If that's too extravagant, then of the two listed I'd pick the Nexus. Being able to run the latest OS is a big plus in terms of longevity.
My String Journey: http://tt.tennis-warehouse.com/showthread.php?t=362403