Bottom Line: Well done, allows more levels to be downloaded and free!
Free apps don't get much better than TinkerBox. Sometimes when I review an app and I type something like "$2.99 is expensive" I hesitate, because $2.99 isn't really that expensive, but apps like Tinker Box remind me why...$2.99 is expensive when there are free apps like this.
The app is very well done, with a great tutorial, and the physics work very well. It might get too complicated for younger children but my six-year old loved it and was not frustrated at all in the necessary trial-and-error. I enjoyed it a great deal as well.
There is also an invent option outside of the puzzle levels if your child wants to expand their mechanical creativity. There are four series of levels built into the game now, including the tutorial, and I would gladly pay a reasonable price for more, as two additional levels are listed as coming soon.
In the meantime, user created/submitted inventions can be added to the app via Safari at http://tinkerboxnews.com/inventions/.