Children’s bicycles are measured in wheel sizes varying from 12 inches to 24 inches. The right size for your child depends mostly on their inseam measurement, so that’s the best place to start.
Adjust the seat
The saddle height should be positioned where the knees are slightly bent when the foot is at the bottom of the pedal stroke. Younger children will feel most confident if they can reach both feet on the ground, while older children will want to be up on tippy toes. Make sure to insert the seat post at least as far as the minimum insertion mark. If you need to insert it above that mark, the bike is too small.
Take it for a test ride
Once you’ve chosen a bike in the proper size range, let your child try it out to make sure it feels right to them. The reach, which is the distance from the shoulders to the handlebars, should feel comfortable and their arms should have a slight bend in the elbow. They should be able to turn the handlebars without overstretching or have the grips hitting their body.
Add a helmet
Don't forget a helmet, bell and a light to make sure your child is safe.