Forgot to mention, we have some wooden strider bikes and a couple of mini bikes with pedals permanently off for them to practice on. Many learn on their own bikes, however, using the pedals off technique.
A couple more tips:
1.
Never push the bike - you may not be pushing it
quite where they're steering it. Put hand on back to help them get going if necessary.
2. Get them into the habit of the "pedal up" start position with the pedal level with the downtube so that they can learn to push off strongly
Oh, all right, three tips
3. Get them into the habit of looking well ahead from the beginning. This helps them to steer straight (ish)
4. (Yes, I know - stop being picky
) Many kids bikes, as we all know, are ridiculously heavy, which is why, in the early days, they may need a push to get started. If the difficulty continues, get your local bike shop to put a larger rear sprocket on until their strength/co-ordination improves to the point where they are spinning their little legs off and you can then reinstall the original.
Once they've really got hold of the cycling bug (or vice versa), don't blame any of us when what you've done really sinks in. Keeping their growing bodies in new bikes will be entirely due to your foolish desire to see them riding a bike.
Good luck