I would highly recommend looking into a travel agent. Many agencies have Disney World specific agents who are extremely knowledgeable on everything Disney and can help you through the process.
Having an agent is especially helpful for a large party like yours is.
I would also recommend to try to split your party in groups of 3-5 people during the day instead of trying to cater to everyone all the time. Obviously those groups can switch people and grow and shrink hour-by-hour and day-by-day, but that might be the best move to keep everyone satisfied.
Beyond that, you can certainly do a lot of research online for everything. However, if you want a "comprehensive" guide, I would suggest going to a bookstore (or Amazon) and buying Birnbaum's Guide to Disney (I would link it, but I'm on mobile). They make a new one every year and they always have a kids version of the book, too. My family used to buy them every year we went and I remember spending hours as a kid looking through the kid's version and the "adult's" version is great for planning things out as well!
One tip that seems to get passed around a lot in this sub with which I agree is to not schedule yourself too tightly. This is especially important with such a large party as there will always be a meltdown or two (from both the kids and the adults) and plenty of things might come up throughout your day that you'll need to adapt to. It's helpful if you allow yourself some wiggle room in your schedules to relieve some of the stress and to allow for unexpected delays in whatever form they may come in.
Basically, I would highly suggest looking in to at least speaking with an agent to get some groundwork settled, then if you want to schedule/plan out some things yourself, I would suggest getting that guide as it will come in handy while you're in the parks as well! You may want to look into some of that pretty soon before you lose some chances to make dining reservations with such a large party.
I hope you have a magical time! (Don't forget to give us a trip report when you're back home!)