A child's room is a miraculous thing and deserves the right sort of decor to appeal to the young one in question - and bringing in colorful and fanciful pictures and posters of cars can be exactly what's needed.
Toy car art is often bright and usually done in paint, sometimes in collage, and often with a playful theme in mind: rather than a grittier depiction of a vehicle these pieces of art often serve as something to bring jolts the child's imagination and entertain them with bright pictures and images.
Toy car art is usually something meant for a child's room, but with the right decoration surrounding it, a poster could be placed along the wall to show an appreciation for art which is simplistic in subject matter but elegant in execution.
When it comes to placing toy car art within a child's room, it's important to consider relative placement to other art, relative placement in the room, and the child's own wants. If the child is still too young to really notice the posters and pictures then you should place the art in a way that will eventually catch the child's attention: not so low on the wall that it can be accidentally torn down and not so high that it will never be noticed.
Remember to take into account what other posters or art you have in the room; toy car art may go well with complimentary colors and themes, but a fire engine may stick out a little too harshly on a pink background.