My experiences with kids at weddings is that they don't do what you expect! Most of the time, i have pushed around the outside of the venue hoping for sleep, and its not happned. If your lo has a nap before you go, i would go with the flow all day...change into comfy clothes/pjs when the evening part of the do gets going (or in the break) and see how long they last. If lo falls asleep on you or in the buggy, then great, if they have interested eyes watching everyone and everything, then just let it happen. If it gets too much, then they'll be sure to let you know!