The Strider bike is perfect for the younger kids who aren't ready for a full blown BMX.
If you want a surefire way to get your kids used to the balance aspect of cycling, a Strider is a great idea.

Features include:

Adjustable handlebar and seat height so that as the child grows and gets more confident you can raise them accordingly.
Lightweight construction - 6.9lbs. This means the bike is more easily manouvred! 
Plastic wheel constructions - no punctures to hold them back! 

From the Strider site - 

"This unique toddler bike has no pedals. With an amazingly low saddle, your toddler simply sits astride the bike , holds the handlebars and rests their feet on the ground. To go forward your toddler pushes with their feet and to stop, they simply put their feet down.

As confidence builds, your toddler will start to stride and then run with the bike, eventually 'gliding' with their feet up on the integrated footrests.

The Strider PREbike™ is designed for toddlers as young as 18 months (as long as they can walk, they can ride a Strider!) up to 5 years. Both the saddle and handlebars are adjustable so the Strider™ grows with your child.

The Strider PREbike™ comes with super-light, puncture-free EVA polymer tyres. There's NO flats, NO pumping air - just get on and go! Weighing just 6.9lbs, the bike is so light that even your toddler can carry it!"