Hello, here are a few ideas that I thought of for your family reunion:
You can make homemade playdough (that will keep them busy for a while), have an egg and spoon race, a bean bag toss game (fill a ziplock baggie with rice or any small noodles and have the kids toss them into a lage coffee cans), a dunk for apples game, musical chairs, the magic ball game, if you are around pavement you can bring chalk and have them be an artist and make wonderful designs, Use the chalk and made a hop scotch, pin the tail on the donkey, or if your out shopping you can look for a parachute (I have one and I bring it out for playdate parties and holiday parties, the kids love playing with it. I think I bought mine at Target or maybe a teachers supply store. It's a smaller version that Gymboree uses).
As for a an icing recipe, I have a homemade one. Use a 2 pound bag of powder sugar, 2 cups of Criso (the white solid), and 1/4 cup of water. Mix together it will be hard at first, but I keep adding a little more water while it is in the mixer, until it's a smooth consistency-not too runny. After mixing, use as white icing or seperate and add different food coloring. When done mixing, spread on cookies or cakes. OR you can go the easy way and buy white frosting in a can and add food coloring to it. The homemade is delicious, but the Pillsbury can frosting is the easiest.
I hope you have a wonderful and funny time at your family reunion!