Help!!

The kids got their first training bike for their 3rd birthday in June. It's August, and they still cannot ride them. Why? Well, they cannot figure out the mechanics of pedaling. They never seemed to grasp the idea on their big wheels, and they still don't now. We're feeling pretty crappy that we can't even teach out kids to pedal a bike, so I'm wondering if any of you have some advice. How did you teach your kids to pedal a bike? Both of our kids push back on the pedals, braking the bike. They can push forward enough to get the pushing leg to the bottom, and then can't figure out how to push with the other leg. **sigh** Major frustration ensues.