We are only a month away from summery Fourth of July celebrations. Each year in the past we are invited to our friend's home in San Diego for a backyard barbeque and to swim in their saltwater pool. It is something we really look forward to each year and of course I always bring dessert. You will see in the link above the cake and cookies I brought two years ago.
You can use most chocolate chip cookie recipes. I use the giant cookie recipe from AllRecipes.com
I recommend using the largest pizza pan you can find. I got mine at Target for a few bucks that I reserve just for the cookie pizzas. Make sure you leave 2 inches of space around the pizza pan for the dough to expand, otherwise it will make a mess in your oven if you don't. Also, I like to put a little extra dough around the edge so when the cookie expands it doesn't get thin and crispy on the edges.
To decorate, gently press some red and blue M&M's into the dough just before baking. Once the cookie has cooled, pipe colored buttercream using a pastry bag onto the cookie.
Above are some chocolate chip cookies my little one decorated. You can also do the same thing with individual sized cookies.