The National SAFE KIDS campaign reports that approximately 18 children ages 10 and under die each year in falls from windows. They also report that another 4,700 children age 14 and under will need treatment for injuries due to window fall related injuries.
All windows within in a home should be child-proofed for window falls. The window screens should not be counted on to prevent falls as they are often flimsy or easily dislodged. You should also remove all furniture near or under windows so that young children cannot use it for climbing to a window.
There are two main approaches for baby-proofing your windows. They are Window Stops or Window Bars. Window Stops are used to limit a window from opening a window more than 4", the maximum safe distance. Window Bars are used on the inside of windows to block access to the window, and come in different sizes. Both are effective deterrents.
The KidCo Window Stop is a good example of an affordable yet effective window stop.
The Guardian Angel Window Guards are the best made window bar protection available.
There are also other options available here.