For children aged around 4-7, we have a used Mexller BMX 20" in matt blue and orange.
It has a steel frame and a single speed, with no gears to shift. There are two brakes: a coaster (foot) brake and a V-brake. The 20-inch wheels have double-wall rims. Mudguards, training wheels, a chain guard and a bell come fitted.
We checked and adjusted it in the shop before sale. After you order, we e-mail an invoice; then collect the bike in the shop or get it by courier.