Get and extra tent and have lots of seating under it. Have the drinks set up under one of these tents and I guarantee that is where the adults will congregate. Like pp said, embrace the rain for the kids. Water balloon/towel toss competitions, squirt guns wars, etc. Buy extra pool towels and have them at the entrance of the house. Put some on the floor (or put down some extra rugs) to mop up the rain/wet dragged into the house when they inevitably want to go inside to change clothes. Tell them to bring a change of clothes for when they are done outside.
Maybe have a craft station on a table under one of the tents for those kids who would rather not get wet and wild if there were other activity options.
" I object to violence because when it appears to do good, the good is only temporary; the evil it does is permanent." Mahatma Gandhi
"This is the ultimate weakness of violence: It multiplies evil and violence in the universe. It doesn't solve any problems." Martin Luther King, Jr.