A good place to start would be to check with everyone to make sure they're comfortable with the idea of a cruise. There is a population out there that absolutely will not go out to sea! And others who are hyper-concerned about seasickness. How many of the group have cruised before? Do any of them have a cruiseline preference? Cabin type preference?
To get a "feel" for prices, check out one of the cruise bargain web sites. Even though your trip is two years away, you'll be able to evaluate how a cruise would compare with some other choice. Remember that all meals and shipboard entertainment, including day activities for the little ones, is included.
But by all means work with a TA. Not only will it not add to your cost, it might reduce it. For the size of the group you're discussing, you should be getting at least one cabin at no cost. A TA may be able to help save money on airfares, too.