I don't know why they put that as the size is more for teen/adult than a child.
Best off getting a longboard with a drop-down deck. It's easier to travel for longer distances as its lower and puts less strain on your legs and feet. I think I paid £40-£44 for mine.
Why does it say child’s? Can an adult not use it?
They are easier to learn to balance on than skateboards so they are good for beginners but they are more for cruising around and not for doing tricks in a skatepark - although there are some good videos on Youtube that show a few tricks you can do. People do longboard dancing but that's obviously for advanced users. It depends on the age of the child - this one would be considered a midsize one so would probably be OK for an older child. My daughter is 6, nearly 7 and she was ok on that Smyths one, which isn't much smaller than this one. Here is a link to a size guide for longboards: https://longboardingnation.com/longboard-sizes-chart/