Hello, Sandy and welcome back to Just Answer!
I have been at Disney World, caught unprepared, in the rain, so I now know how to prepare.
There are a lot of indoor rides/attractions that you can have fun on, even if it's raining. If it's an electrical (thunderstorm), some outdoor rides will be shut down for safety reasons. Yes, you do stand in line in the rain (many have 'overhangs' for protection from the weather), but since it is now the rainy season in Florida (however, it can rain on any given day) bring rain gear (rain ponchos/small umbrellas and a plastic bag to store them in before and after use); they also sell rain ponchos and umbrellas in the theme parks (of course the prices are quite high). It would be worth it to rent a locker to stow this stuff after use. If you get caught in a downpour, it's always good to bring a change of clothing or at least footware (socks/shoes) for all. Many times, the summer afternoon downpours are shortlived, and you can still take advantage of enjoying the outdoor rides/exhibits. However, if it is a predominantly stormy day, there are many things you can do in the area for fun and to keep a 6 year old entertained.
You can take the monorail to the various Disney resorts where there are fun things to do. The Polynesian, Grand Floridian and Contemporary resorts are great. The Contemporary has the most arcades. You can find interesting shopping in Downtown Disney and Epcot's World Showcase and there are many things to keep kids busy. Yes, it might cost a little more than you expected, but under the circumstances, it's definitely worth it. Of course, there are also wonderful restaurants throughout the theme parks and at the resorts.
If you're planning on renting a car, you can also visit Busch Gardens which is not too far away. While they have shows outside with the dolphins, whales, etc., there are also indoor exhibits with aquariums, etc. to keep you and your child busy.
You can find more information here:
I hope they don't have any rain on their vacation, but if they do, you're very wise to help them start planning ahead! I hope they have a great time!
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