A day at the beach with kids can go spectacularly well or completely sideways, and the difference often comes down to what you packed before you left the house. Forget the snacks, sunscreen, or goggles and someone is miserable by noon.  Overpack and you spend half your time hauling gear across hot sand while a toddler trails behind. The sweet spot is somewhere in the middle: everyone has what they need without lots of extras for you to keep an...