To find out if the mirror is easy to fit on a 28-inch BTWIN children's bike, it's important to check that the handlebar diameter is compatible. The Busch & Müller Cycle Star E mirror is designed for handlebar diameters ranging from 14.5 mm to 22 mm. If your bike's handlebars fit within this range, installation should be relatively straightforward. If not, you may need to adapt or choose another model.