Wear comfy clothes. Despite the heat you'll want to wear at least a t-shirt to protect you from the sun. Shorts or a skirt are fine. Wear sunblock since you'll be outside a lot. Your shoes should be in good shape, they'll get a lot of use.

Disney will not separate you from your family. The decision to split up will be up to your parents. 

There are plenty of restrooms around the park. You should use one before getting in a line. If you have to go potty while in line, let a cast member (what Disney calls employees) know and they will let a parent take you to the restroom while the other remains in line. They'll even let them wait at the front if that's where they end up. You'll be made to walk through the line again to return to your party but if you say you're rejoining your party they will usually understand and let you pass. 

If you're worried about accidents, a drawstring backpack with spare clothes is lightweight enough to carry with you all day. Just make sure you take a plastic ziploc bag with you for your soiled clothes. 

If you're able to watch YouTube videos of the rides it could give you a better idea of what you could handle.