How do I open the bottom cover?

I'm trying to repair/clean my son's Dickie Toys Lightning McQueen RC car, but I'm having troubles getting the car dismantled. There are three visible screws in the bottom (one under the batteries), but even after unscrewing all of them, the car body does not detach.

There are two weird-shaped holes in the bottom that look like they would maybe fit a T-shaped key about 5-7mm wide (and around 1-2 mm thick). Am I correct in assuming that they are used to attach/detach the plastic parts and that there is a special opening tool for these?

I could try to mock something up from a piece of plastic but I'd like to get some more information before I risk my fingertips with my crappy dremel.

An imgur picture of the holes: http://imgur.com/hiKDlDK