The key to packing lightly is to ensure that you take along everything you need but leave behind anything you don't. To achieve this, I brainstorm and make a list of everything I might need. Then I cross out anything I can survive without for the duration of the hike.
Don't leave behind vital medication, a first aid kit, food, water, and waterproof layers for higher altitudes. If any member of your family has allergies, an EpiPen is essential. Pack heavier items into your backpack first and arrange them so it's balanced. Anything you might need in a hurry, like that EpiPen, put into side pockets or pack on top of everything else.