Smaller is Better than Bigger for Kids Learning to Ride Bikes
Almost every parent I have come across likes the idea of buying a bike that their kids will grow into. Of course this makes sense financially, but there are negatives.
The negatives of a bigger bike are that the extra size, height and weight are quite daunting, and often a child can't actually properly ride the bike or safely control it. Where's the fun in that?
Smaller bikes (that is the smaller size of the two practical options) are more fun; the kids have more control and they feel the master of the bike. They are far more likely to ride a smaller bike, and ride more often, plus they learn bike-handling skills far quicker and to a higher standard. Financially if you want to make sense of this, get two kids onto one good (smaller) bike.
What size fits? It is too easy to use height as the only measuring stick for a bike's correct size. Be careful: it can easily be misleading as an indicator.
As most kids' balance develops at roughly the same speed, using age with ability is far more relevant. If you can add age, with height, and ability (ability is a hard one to measure but as a parent you must be honest and realistic) and confidence, than you have a really good indicator. As an example, a confident six-year-old rider who is small can cope with a slightly bigger bike better than a taller five-year old with undeveloped skills.
One last consideration is the "hand me down factor". If after all elements of height, age, ability and confidence are considered, it's best to go small and hand the bike down to the next child a little sooner.
Warren Key, Melbourne bike retailer and designer of ByK Bikes for Kids