The sub-category 16 "and 18" designates the wheel size of kids' bikes. Here you will find hundreds of listings and ads of new and used bicycles, of the most varied types, of bikes with dimensions suitable for 5-6 years (16 inches) and 6 to 8 (18 inches).
They are solid bicycles, with bright colors, similar to those of adults, obviously with pedals but fixed ratio. They are equipped with mudguards and bell, and many have side wheels mounted to help stability when the kid is still learning. Once he/she rides safely, you can easily remove the wheels.
Regarding the typology of the 16 "and 18" bikes, there are pretty bicycles for girls in white, pink or fuchsia, with or without basket, captivating two wheels for boys or bikes with shapes inspired by MTB, which look like adult vehicles, but in miniature.
EurekaBike is a large bicycle marketplace and covers many manufacturers around the world and authorized dealers. You will find offers of kids' bikes from the best brands (Atala, Trek, Scott, Specialized, Early Rider), as well as many ads for second-hand kids' bikes.
You can compare prices, data sheets, manufacturers. Find the bike online at an affiliated retailer, or choose the shipping (safe and tracked from all over the world) to your home, or to that of the kid, perhaps for a surprise.