The best size bike for a 10-year-old child is generally a 24-inch wheel bike. However, its important to keep in mind that children grow at different rates, so its best to measure your childs height and inseam to confirm the correct size. The inseam length is the most important measurement to consider when finding the right size bike for your child. The child should be able to straddle the bikes middle bar with their feet flat on the ground, and there should be 1" to 2" of clearance. Additionally, while pedaling, there should be no knee bumping into the handlebars, and the child should be able to pedal smoothly without hyperextending their legs.