Here in the midwest we have been dealing with the remnants of Hurricane Isaac. I know we got off easy compared to some other areas, so I'm very thankful for that. Our home field was a disaster after getting 5 inches of rain on it, so a local high school with turf was willing to let us play there. I have to admit, it wasn't as bad as weather.com had predicted, but I'm glad I prepared accordingly.
3 changes of clothes complete with underwear
1 8x6 tarp
large body warmers (in case it was a cold rain)
first aid kit
bug repellant (we have had west nile virus identified in our area)
Over all, the kiddos (5 and 7) did great, they thought the panchos were awful until the rain started and then they realized wearing them insured dry clothes.
We spread the tarp on the bleachers, it was long enough to pull up over our backs to protect us from the rain AND cover our bags underneath our feet.
We didn't even need the extra clothes, but I'm glad we had them just in case!