To stop a child from opening a window, window latches are a good choice. Just make sure that you choose a type of latch that will not only stop a child from being able to open the window but at the same time is easy for an adult to do so.
If you want to prevent a child from gaining any access to a window then window guards are a good option. Window guards normally come with bars that extend across the window, preventing a child from reaching it. While these devices should all comply with modern safety standards, it’s a good idea to check for yourself that bars are not spaced more than 4 inches apart which should ensure that a child does not trap their head or another body part in the device.
What if you want to be able to have the window open? In this case, window stops are the best choice. These allow for the window to be open to a set width. Make sure, however, that the set width is no more than 4 inches.
The way you position furniture will also help in preventing a child from reaching a window, be sure that furniture items cannot be used to climb on and access a window. Another important thing to have in mind is that window screens are not childproof, if your windows are fitted with screens, they will still need to be fitted with childproof devices.