Some of the most common summertime ER visits are for sprains, strains, open injuries and head injuries. The main thing to prevent these types of injuries are prevention, properly training children, close supervision and just plain-old common sense.
Some things smaller children should stay away from are ATVs, motorcycles, anything that gives height and travels fast. These are much more dangerous when the child is not supervised and trained properly. Also trampolines are a common summertime cause of injury.
Properly train your children to use the equipment right and to always use safety equipment and helmets (when required). When children and parents properly use these things, it is less common to see them in the Emergency Room and it definitely minimizes the severity of the injury. Lastly, just use common sense, if it sounds unwise then it probably is.