The sight of small children struggling with trainer wheels on a bike too big for them is unfortunately an all too common sight and certainly not a good introduction to enjoyable bike riding. The next stage in the process is usually a parent running along almost bent double attempting to steady the child by holding on to their bike seat, again not the best way to help your child ride or your lower back.
The earlier we show our children that cycling is not only a lot of fun, but incorporating it into life on a regular basis provides a solution to many environmental, community problems. Teaching and encouraging your children to cycle is the best sort of activity, suiting the "THINK globally, ACT locally" concept perfectly.
First of all as a parent lead by example use your own bike as much as possible, then by the time your child is four or five years old the concept is familiar and they are usually anticipating being able to cycle with you. Equip your child with the standard tricycle (and helmet of course) this will hone their peddling skills to perfection far better than a bike with trainer wheels as they tend to be heavy and unwieldy. The 'fixed' wheel drive too I believe adds a sense of control, most children will quickly be able to stop and even do three point turns. Their first two-wheeler must obviously follow that often repeated piece of advice and be the correct size or even perhaps very slightly small, please do not be tempted to buy the next size up as the child will usually find the bike to unwieldy to use successfully. This is very important to ensure the child is successful, safe and enjoys the learning process.
In my experience once a child has learnt to ride a small bike with confidence they out grow it very quickly, and are then ready to move on to the next size up. A child who is keen to learn will very likely be riding a two wheeler within a day if the following instructions are followed. To set up the bike for the child to begin learning to ride, first remove the pedals and lower the saddle to a point where the child is able to sit astride the bike with both feet flat on the ground. What you have now is very similar to the running machine as invented by Karl Friedrich Drais von Sauerbronn in 1817.
The aim is for the child to freewheel confidently with their feet lifted off the ground so achieving the essential steering and balancing. Once they have such control and are starting to pick up a bit of speed put the pedals back on, find the largest flat sealed area you can and they are almost ready to be let loose.
The trick to effective help at this now is to fix a broomstick or similar lightweight pole to the bike to act as a handhold, enabling the adult to have complete control over the balance and speed of child and bike. It also means the adult is upright so can run easily beside the rider avoiding the back braking stumble holding the back of the saddle. The broomstick is placed down between the seat stays immediately behind the saddle and wedged between the chain stays behind the bottom bracket then fixed securely in place with cord or duct tape. By keeping a loose grip on the pole you will feel when they are riding but have complete control over the bike and rider. Very quickly they will just ride out of your hand and you will have the satisfaction of having taught your child one of the most important things they will ever learn.