I would have voted for Kids of Carcassonne if I'd been paying closer attention to the General forum.
The tiles are big and sturdy, and the backs are the most interesting of all the Carcassonne spin-offs in my opinion (which
jungleboy will inevitably disagree with purely for the sake of it

). The game is very simple, but it's still fun to play every now and again, and I like that my daughter (5 y/o) and I can play head to head without me having to make any bad decisions in order to keep her in the game.
With a game like this, I can imagine it being very easy to make it simple, but boring at the same time whereas I don't feel that this is something that this game suffers from. It certainly is simple, but the tactical/ strategic element is still there to a degree and is easy enough to grasp that even my daughter recognises how to place the tiles in order to maximise her chances of completing her roads without adding to mine (we always play this head to head and have two colours each).
I've never really played The Castle but I've watched it being played and nothing about it appealed to me apart from the fact that it was 2-player only.