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The first thing to do is measure your child's inseam (that's a fancy word for where the legs meet the hips). If that's too socially awkward for you, just measure up to the bottom of their butt. You want the bike to be about an inch lower than the inseam. This way it will be comfortable and still have a decent life span (as far as kid's bikes go). Most stores provide a bike height measurement that their inseam height will correspond nicely to. - 2
Next, if possible, take them to the store to try out the bike you are considering. Have them sit on its seat and see if they can touch the ground. For comfort and safety reasons, you want them to be able to touch the ground, at the very least they should be able to balance the bike on their tippie-toes. Bike tires aren't really an issue in measuring. - 3
Now have them lean from side to side and see if they can hold up the bike with just one leg. If they can't it probably means the big is too heavy (and too big) for them. Make sure they can reach the handlebars without too much trouble. If your child is bending their elbows and knees, that means the bike is too small. Try to find a bike with a nice balance. - 4
Finally, pay close attention to the seat of the bike. Its material and height are both important. If your child can reach the handlebars but can't touch the ground, see if the seat can be adjusted. Find a bike with an adjustable seat, this will greatly increase the life span of your purchase.
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