Catherine,
While I think that the fireworks at Epcot are the best on property, I think that the Magic Kingdom fireworks are better to watch from the adjacent bodies of water.
Now make sure you have the appropriate reservations before you arrive. Many of the marinas in the area close well before dark, so you may come here and find yourself unable to rent a pontoon as planned. A good example of this is the Grand Floridian marina that closes at 5 pm.
I would recommend that you call 1-407-WDW-PLAY and ask about what your options are. If you can't find a simple rental that meets with your plans, I would recommend you consider the
Pirates and Pals Fireworks Voyage. I did this with my girls two years ago (when they were 3 and 5) and we all had a blast. It is not just for younger kids. It was fun, entertaining, and quite scenic. The pirates that lead the cruise are VERY entertaining, and it is more than just going out, watching fireworks, and returning to shore. It is expensive, but we thought well worth it!
Write me back if you have any follow-up questions!
Have a magical vacation!