Screens are flimsily made; they are a weak barrier and give a false sense of security. They are great for keeping insects out but are not strong enough to keep children in.
Follow these recommendations to prevent falls from windows:
Links
[1] https://www.thechildren.com/trauma/en/news/news.aspx?id=592
[2] http://www.montrealgazette.com/travel/Toddler+hospital+after+three+storey+fall+downtown+Montreal/4977804/story.html