Check the weather forecast for Anaheim, California. Knowing what the projected temperature will be during your visit will allow you to bring the right amount and weight of clothing.
Dress in layers to face any temperature. It will be cool in the morning and evening on even summer days, so wear something like a sweatshirt or jacket you can remove and tie around your waist. Take advantage of lockers to the right of the entrance to stash bulky clothing like jackets that you'll only be wearing at night.
Wear comfortable padded shoes suitable for walking on pavement.
Bring a large backpack or tote to carry essentials (like your wallet, suntan lotion and maps) and camera equipment. You'll probably also amass souvenirs, bottles of water, sweatshirts and prizes from midway games along the Paradise Pier boardwalk as the day progresses.
Apply sunscreen and bring a tube for reapplication throughout the day. Wear a hat and sunglasses to protect your skin from the sun even on overcast days.
Carry a change of clothing for the kids if they'll be playing in 1 of the 3 available water spout areas in A Bug's Land. With water drizzling, pouring and shooting out of the ground and pipes, they're sure to get soaked.