ZT2 is not childish, but Roller Coaster Tycoon 3 is another excellent choice. It's very similar to Zoo Tycoon 2, but with Roller Coasters instead of zoo animals. Basically, RCT3 and ZT2 are two of the hottest simulation games and they both have many similarities.
I have really enjoyed RCT3, as have my daugthers. It's a blast to ride the roller coasters in 1st person. It's a really well done and immersive simulation game. I also have the Soaked! Expansion which adds the whole aspect of a waterpark with slides and pools.
One thing about RCT3, it does require a fairly spiffy video card. The game has night time scenarios and low end cards get a horrible framerate during the night scenes. I was getting around 1 FPS with the integrated Intel 950 and 128 MB shared on a laptop. Not a very great 3D gaming card, but FPS is the #1 complaint about the game.
I hope she enjoys it.