I am pretty sure you would all fit on a pontoon boat (I believe 10 is the maximum for those). The smaller canopy boats would not fit everyone in your party. They also have paddle boats that seat 2 and I believe they allow one child to sit on the back
I do not remember rental rates but you could call and ask.
Old Key West 407-827-7700
Fax - 407-827-7710
This is the front desk number but just ask for Hank's and they can give you the rental rates.
Personally if I was renting a boat for a group I might be tempted to go over to the Contemporary or Ft Wilderness and rent a boat from ther...so you can have a lot more scenic trip with more space to cruise. At OKW there is only the canal going to DTD that opens into the lake there at DTD which isn't too big. You can take the canal down by POR and POFQ and it is a scenic trip on the canal, but it is still a canal