K.U.
Get the Unofficial Guide to Disney World - they come out with a new edition every year. All the most helpful advice and information you could ask for. It's been required reading for us every time we have gone.
When we went this past February, we actually rented a house for the week and that worked best for us. We went grocery shopping, cooked our own meals and saved a ton of money on food by not eating in the parks all the time. We had our own pool and hot tub. Kids had their own rooms. We did have to drive to the parks every day and pay for parking but it was still worth it for us. Since you have a bigger group, that's something you might want to consider.
BTW, Harry Potter is at Universal Studio's Islands of Adventure - not Disney. If you do go there, get there EARLY. Like, before they open, and make a bee line for Harry Potter. It's hugely popular and the long lines form fast.