Hi Connie,
Personally, if you are just heading over there for the
Pirates and Pals Fireworks Voyage, I would drive yourselves. When taking a bus, you need to give yourselves at least an hour and a half. This is because they come approximately every 20 minutes. You never know if you just missed one, or if one is coming around the bend. Then once you get to the Magic Kingdom, you would need to either hop on the monorail to the Contemporary or walk over there from the bus depot. Top that off with the fact that you don't always get a seat on the bus or monorail and you may end up having to stand. With 2 little ones, it can get tiring, especially later in the evening.
When you drive to the Contemporary Resort, you'll simply tell the gate attendant that you are there for the Voyage and they will tell you where to park. Since you need to arrive by 8:30pm, and you need to locate the ballroom inside the resort, I'd shoot to get there by 8:15pm instead. Therefore, I would plan to leave the Caribbean Beach Resort by 7:45pm at the latest.
Wishing you a wonderful vacation! Have fun on the Pirates and Pals Voyage, it is so much fun!
Kaylene