Joining an iPad-hosted game from Android
Solution 1:
Yes, according to the MinecraftWiki.net:
"The multiplayer mode is cross-platform compatible between Android and iOS."
Most people have said it works fine but I have heard of a few rare situations where one could not successfully use an Android to connect to iOS. Perhaps it was due to some user error or something else, it's hard to say. Hope this helped.
Solution 2:
Yes, this is perfectly possible as long as they are both connected to the same wifi network. We do this at home all the time. The game is started on the iPad and my Android mobile phone (HTC-One X) can connect and join in the world quite happily.