KOA is a fantastic franchise! Call ahead and ask what activities, food, & etc they have. Nearly every KOA I've ever stayed at (many since I was young, and all across the country) has had very friendly and helpful staff. You'll probably even get some good pointers & information if you just tell them "it's our first time, and we have 2 kids"! I'd say to go minimal on the toys/entertainment.
Bring extra blankets, towels, and clothes. Nothing worse than climbing into bed and feeling all itchy from the sand/grass/etc! Make sure your tent is big enough for all of you! Or better yet, look into a Kamping Kabin! They aren't much more expensive than a tent site, but are more secure, have beds, doors, and electricity!
You can just do "picnic" foods (sandwiches, fruit, granola bars, poptarts, etc), if you think it will be too much hassle to cook. I do like to cook "camping meals" (hot dogs, mac n'cheese, burgers, corn on the cob, etc), but it adds a whole extra level to the preparations (don't forget the milk, butter, ice, etc). So, like I said, for the first time, I'd stick with No Cooking. This goes along with calling the campground ahead of time, too, because many KOAs will have group meals and snacks (pancake breakfast, ice cream social, hot dogs & chips, etc).
Most of all--just Have Fun! Camping can be a wonderful experience for your family! I've always enjoyed camping and take my daughter as often as I can!