Be vigilant! Sometimes accidents happen to children when we assume that someone else is looking after them. During family gatherings, designate one adult to be in charge of the children. Make sure this person knows exactly what his responsibilities are.
Identify the adult in charge! For example, have him wear a colourful and playful accessory such as a hat and then when his supervision duties are over, ask that he pass the hat on to the next person who will take over and so on and so forth!