This is a typical trailer device, which attaches to the back of an adult bicycle. They are available with 20 inch wheels for smaller kids (up to 45 inches tall) and 24 inch wheels for bigger kids (40 to 52 inches tall.) There are several brands which work well, all of which have a quick-connect system, so that you can ride your bike with or without the trailer. They fit most bikes. Our rental trailabikes are set up to fit only 27.2 seatpost, which we have on most of our rental bikes.