Final answer:
Speak to the parents of the young girls about the safety risk of sitting on the windowsill and offer suggestions for childproofing to prevent potential falls.
Step-by-step explanation:
If you observe that the young girls next door, ages 2 and 3, often sit on the windowsill with their legs hanging over the outside edge, the best course of action would be to first talk to their parents about the safety concern. It is important to do so in a professional and friendly manner, and you may wish to cite safety recommendations that young children should never be left unattended in areas where they are at risk of falling. Besides talking to their parents, you can offer to help by providing resources or suggestions for childproofing their home, including the installation of childproof locks on the windows to prevent future incidents.