When finding the perfect bike size for your kids, it can be confusing. There are several things you consider: rider height, bike wheel size, frame size, etc.
Here's a guide from the Two Wheeling Tots that will help you in finding the perfect bike size for your child.
There are a several factors for sizing, but the BEST FIRST STEP is to measure your child's inseam height, as this will be compared to his age, and finally, the frame and tire size (which works together to come up with the seat height of any bike).
Here are the 4 STEPS from Two Wheeling Tots to help you decide on the perfect fit:
Finally, here's another table chart to guide you through your selection of sizes if you can't find match your "perfect range" on the chart above.
Just measure his height in inches or cm, and find your perfect fit.