Plan Ahead and have on hand.
Make sure to have a few extra seats and dinner setups; plates, flatware, glasses, and napkins for any surprise guests who may show up. This happens more often than you would think. Last wedding we were partook in was at a rented facility that everything was brought in and we had six extra sets (including chairs) and ended up using four for unexpected plus ones and the other two for little guests who just so happen to drop their items on the floor. Always a great idea to plan a few so no one is caught off guard by these moments. We were able to add these extra guests in with ease and even help the little ones out who lost their flatware before the meal began.