As a guest I had the most wonderful time in the parks when it rained. If you're not afraid to look a little silly, you're good.
Roll up your pants so they don't get wet, wear a poncho with a baseball hat underneath and a good pair of shoes and go play

No, seriously, I know you'd prefer to have picture perfect weather and not a cloud in the sky, but the worst thing you could do is worry about it too much. Just be prepared for ANY type of weather and it won't be so bad.
I once went to the park with my friends and the weather was PERFECT, no indication of rain, nothing. And we should have known better than to go without umbrellas, hats or ponchos in July....but there we were, as all hell broke lose and it rained so hard, we didn't dare to look up in the sky, because we thought we might drown instantly. We were completely taken by surprise and went into fits of giggles and ended up just playing and dancing in rain, jumping puddles and just having a grand old time. Needless to say that a Margharita or two was involved. My point is: You can still have a GREAT time, even in the worst of all rainstorms.
Just make sure to hit the parks that are heavy on parades/shows, like MK/Studios, on a day that is most likely not going to have that much rain, since the parades/shows are pretty much the only attractions that will be affected by heavy rain.
Have fun
