First I would suggest not to show any intense emotions. A child might look at the reaction of an adult to understand how bad things are. So don't make them scared of guilty about the situation. Offer them a help to get up, then ask to look at their face. Show some compassion, but don't make them feel as their injury is too bad. You can also offer a disinfectant and a medical plaster, if you can get them quickly (for example it's in the first aid kit in your car). Ask where are their parents or other adults that are watching them, and bring a child to them. 
I assumed that there are adults watching for a kid. Because a child left unattended on the street is a separate problem.
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