Getting all your stuff down to the beach is a cinch with a wagon. Buy one with wide, all-terrain wheels, then load it up with beach chairs, towels, sand toys, sunscreen, and a cooler filled with lunch and drinks.
Make sure you reapply SPF 30 every two hours throughout the day and each time your kids come out of the water. You should do your first sunscreen application at home, before they put their suits on.
You probably won't be able to keep a hat on your kid for the whole day, but try to get her to wear it as much as possible. Broad-brimmed hats are better than baseball caps because they protect your child's ears and the back of her neck too.
Have kids drink enough water during the day so they don't get dehydrated. Freeze a few of your water bottles before you throw them into your cooler -- they'll keep your sandwiches fresh.
Store all your sunscreen in a zip-top bag. It'll be easier to find in your beach bag, and it won't get sandy.