Keep them closed. Toddlers and older children are attracted to windows, especially when open, so they can be a danger zone. For the 2-6yr group, they are creative at moving ottomans or chairs up to windows to see out, they lean on a screen not understanding it won't hold them & fall out. Either keep windows locked or have locks that prevent opening more than a few inches and store easily moved furnature.
Approved bars.... ..Are probably the best way, but also having nothing near windows that the child can use as stairs is a good idea.