Planning a 3yo Birthday Party

A few weeks ago it occurred to me that I had to start planning a 3yo birthday party. I booked one of the children's singer at we saw at the library. When he asked who would like to come up and sing. She ran to the microphone and sang itsy bitsy spider.

We have about 20 children coming. I was hoping to have the concert in the yard. I'm a little concerned now. We are expecting rain on Sunday. Luckily we have a large art studio above the 2-car detached garage. We haven't cleaned it since we moved in and it's pretty dirty. I started moving our stuff out and scrubbing. It's filthy. It's going to take a few days.