Teach a Child to Ride a Bicycle Quickly

How are your kids doing when they learn to bike without stabilizer wheels?

They need someone to grab the bike to prevent fall down, right?

Are you already tired to run with kids supporting their bike?

Steps

Remove the stabilizer/training wheels from the bike. If they stay, it is a bad habit for them to rely on them. If you keep them on, try to slightly angle them upwards so they do not disrupt riding, but are there in case of a fall.
Remove the pedals. While doing so make sure the bike cranks will not have loads of grease on them or have any sharp surfaces.
Let child ride and push the bike by their feet and practice balancing. Make sure they are sitting down and not walking.
Let them practice until they are comfortable and can coast long distances without touching the ground.
Put pedals back on the bike.
Say this to your child when they get on the bike. "Pretend your feet are stuck to the pedals with superglue, don't take them off". Also tell them to coast if they go too fast for they might injure themselves if not educated about how hard to press on the brake.

They maybe need to get the point of how to push pedals, but it should be easy after this process.
Make sure your child has some type of ID on them when biking alone or around the neighborhood. Or something as simple as a piece of paper with their name/parents' names and home phone number will work just fine. This can easily fit in a bike pouch under the seat. Communication is vital if your child gets injured or lost.

Never ride without a helmet.
Even they can start this easily, they easily fall down too. So, of course someone need to support sometimes. Especially when they lean how to keep pushing pedals to proceed long run.