Hmm, strange. The bars don't have any kind of close button, so are you sure it didn't just get moved somewhere else?

I poked around a bit, but I'm not sure where the editor saves its config file for window layout, so I don't know how to reset to defaults. You could try installing/reinstalling, but the config may be saved someplace that an uninstall wouldn't remove.

In any case, you can still add Goo Balls to the level. In the "Level" tab on the right, right-click on the "level" element at the top and choose "Add child Ballinstance". You can specify the Goo Ball's type manually.

I had a look around and the bars can actually be closed, or to be precise, they can be disabled. If you right click on any of the bars, a menu will come up and you can enable or disable any of the bars.

Most likely your son had accidentally clicked on "Add Goo", removing the bar. Just click on it and it'll pop back up again