Riding with your children is an awesome experience. It's a great way to spend family time together and get some exercise.
Getting a good fit is the most important part of finding a helmet for your child. Pop in to your local bike shop to find the right one.
Choose interesting places to ride with your children. Include a cafe stop and somewhere to practise jumps and other skills. Travelling speed with children on bikes is generally pretty slow, so bear that in mind when planning your route.
Chances are your children love riding their bike, so all you really need to do is facilitate that. Their ability to judge distance and speed is generally less developed along with limited peripheral vision.
It's best to carry a lock with you at all times so if everything goes a little pear-shaped, as it can with kids, then you can just lock your bike and pick it up later.
Pedal Ready and WORD are great places for your children to learn how to ride.