How do I remove rear wheels...
 

...on an old Tyco Aero Hopper Turbo?
I was given one, and it looks like fun for the boy (10), but there is enough play in the rear axle for the gears to slip, so I want to check out the gearbox.
Each rear wheel has a hex-nut at the center, but it isn't obvious which way they turn. They seem to turn in the same direction, as holding one wheel and trying to unscrew the other doesn't work. I've gotten the gearbox, axle and all, off the car, but I still can't see a way to secure the axle for removing those nuts.
Any clues would be welcome!

Thanks in advance!

Alex

RE: How do I remove rear wheels...
 

Hold both wheels and turn nut ccw?