As Robert the Guitar Guy, I’ve done literally thousands of performances. In so doing, I’ve learned not to panic when I don’t have something that I think I need for the show. I’ve even done a show without my guitar! Imagine Robert the Guitar Guy without his guitar!
Once my car broke down when I was doing a summer tour. I secured a ride to my next library gig but the tow truck took the car away – with my guitar in it! There was no time to track down the guitar. I called the library and told them my predicament. Just before I went on, someone handed me a twelve-string which is impossible to play in the finger-picking style I favored then. The kids still really enjoyed the show. It was me, not the guitar!
Same with your party: If at the last minute you remember that you don’t have the goody bags or the finger food or the piñata, don’t panic. It may be hard to believe but you are the most important element of the party!