Both my old iPad 3 and 4 got slightly warm while charging which I believe is normal. However, it shouldn't getting too warm. If it is I would make an appointment with an Apple Genius and get them to look at it just to be sure.
Some heat is normal my i5 didn't warm up at all but my i5s gets warm when i play lengthy games and if I have it plugged in while playing the charging block will also get a little warm