How to measure bike size for kids effectively?

To measure a bike size for kids effectively, have the child straddle the bike with both feet flat on the ground. There should be a 1-2 inch clearance between the top tube of the bike and the child's crotch. The child should be able to comfortably reach the handlebars and pedals. Adjust the seat height so that the child's legs are slightly bent when the pedal is at its lowest point.