It's all about personal preference, right? What do you and your daughter think? We'll each have our opinions about what looks right, but in the end it's you that gets to enjoy the tank each day, and if you like it like this, that's what matters.
Now that I've said that, I'll interject my opinion. lol Personally I think it's too big, but I wouldn't want to risk destroying it's character or that little cave, as I don't suspect it'll come apart easily. It's a very cool piece as it is.
instead what I would do is save up a little bit and get a larger tank, I think that piece would look amazing in a 20 gallon. haha!